There already is an Echo Knight mod by lostsoulman. I didn't find it when searching before starting work on this (its name was different then), and when told about it I was already hooked on trying to implement Echo Knight as close as possible to the D&D rules.

Lostsoulman's mod is an excellent representation of the subclass implemented natively in BG3.

This mod takes a deep dive in the weirdness that is the RAW 5e Echo and is more streamlined to use, but is entirely dependent on Norbyte's BG3 Script Extender as much of its functionality is either impossible or impractical without it.

The two do not conflict if Compatibility Framework is used, so you can install both and respec back and forth.

Echo Knight

A mysterious and feared frontline warrior of the Kryn Dynasty, the Echo Knight has mastered the art of using dunamis to summon the fading shades of unrealized timelines to aid them in battle. Surrounded by echoes of their own might, they charge into the fray as a cycling swarm of shadows and strikes.

Source: Explorer's Guide to Wildemount

Manifest Echo

At 3rd level, you can use a bonus action to magically manifest an echo of yourself in an unoccupied space you can see within 15 feet of you. This echo is a magical, translucent*, gray image* of you that lasts until it is destroyed, until you dismiss it as a bonus action*, until you manifest another echo, or until you're incapacitated.

Your echo has AC 14 + your proficiency bonus, 1 hit point, and immunity to all conditions. If it has to make a saving throw, it uses your saving throw bonus* for the roll. It is the same size as you, and it occupies its space. On your turn, you can mentally command the echo to move* up to 30 feet in any direction (no action required). If your echo is ever more than 30 feet from you at the end of your turn, it is destroyed.

• As a bonus action, you can teleport, magically swapping places with your echo at a cost of 15 feet of your movement, regardless of the distance between the two of you.
  

• When you take the Attack action on your turn, any attack you make with that action can originate from your space or the echo's space.* You make this choice for each attack.
  

• When a creature that you can see within 5 feet of your echo moves at least 5 feet away from it, you can use your reaction to make an opportunity attack against that creature as if you were in the echo's space.*
  

* Translucent
Starting with some nitpicking, but the Echo is not translucent. Counter-intuitively, I felt translucent effects were distractingly attention-grabbing. The Echo uses customised VFX to look like a desaturated unstable image of the Knight though.

* Magical Image
Much of the complexity of this subclass is merely implied from the description of the Echo as a "magical image." The Echo is not classified as a creature but an object in the D&D rules, which also means it can't be an ally or enemy. See this excellent FAQ for clarifications and implications, but in short it means the Echo can't be targeted by most spells and doesn't interact with many class features.

An entity in BG3 can be a character or item, which generally maps well to D&D's creature and object concepts... but a BG3 item can't look and act like a character, which the Echo needs to do. To get around that issue I made the Object Character Support mod, which handles most of the messy bits of making BG3-characters that act like D&D-objects.

The Object Character Support mod is not strictly required, but without it the Echo will not work as intended. It will still work and will be a bit more like any normal summon in BG3.

* Dismiss
Dismissing the Echo is a free action on the Echo, rather than a bonus action on the Knight. Mostly to avoid cluttering the Knight's bar, but it also matches how many similar features are implemented in BG3.

* Saving Throws
The Echo has the saving throw bonuses of a single-class Fighter of your level with your ability scores. The possible discrepancy is almost entirely irrelevant, especially when using the Object Character Support mod. Few things that cause saving throws can even interact with the Echo, and fewer still that don't also deal the 1 HP of damage required to destroy the Echo either before the saving throw or on a successful save.

* Movement
In D&D, the Echo doesn't have its own initiative and is only moved on the Knight's turn. The Knight is given a "Move Echo" free action to do this, but proper previewing of movement in BG3 is only supported for a character moving on its own turn, so the Echo is also given a turn and ability to move.

The Echo gets a similar Dexterity-based initiative bonus to the Knight, to make it somewhat likely they end up with shared initiative, but it currently doesn't seem to be possible to force shared initiative. The Echo's movement can be divided freely between the Knight's and the Echo's turns, and both share the same movement speed resource pool. When on the Knight's turn, the Echo's remaining movement is displayed as the duration of the "Echo Movement" buff applied to the Knight.

If the Knight tries to order the Echo to move further than it can reach, you get a ping at a location the Echo can reach in the same direction assuming there are no obstacles in the way.

Note that the Echo "flies" as there's no restriction on which direction it can be moved. Visually it's standing on the ground while stationary, but imagine it floating just above. Because it doesn't need to touch the ground, it's unaffected by all surfaces in BG3. It also does not provoke opportunity attacks as those only work against creatures.

* Attacking from the Echo's space
Attacking from the Echo's space is the other complicated part of the subclass. The Knight gets two different ways of doing this, with different tradeoffs.

Both methods are blocked if you are blinded, as it's still the Knight making the attacks using its own senses from its own position. Shoving from the Echo's space works if you use a mod to turn shove into the special attack action it's supposed to be - such as Shove as an Attack or Combat Actions - but standard BG3 shove doesn't qualify.

Control Echo: Activating will instantly swap places (and near-instantly swap VFX) with the Echo, to give the illusion of controlling the Echo to make an attack. This is the best representation of the feature as you actually are in the Echo's space making the attack.

You're invulnerable and immune to surfaces and some persistent spell effects while swapped, to avoid effects the Echo wouldn't be affected by, but this is not a perfect match for the Echo's protections and non-creature nature, so swapping into dangerous situations carries some risk.
The granted immunities can also end currently applied effects, which is not intended, so to prevent exploits you are not allowed to use this feature while afflicted with certain conditions.

Attack Near Echo: Alternatively, you can remain in control of the Knight but enable attacking targets around the Echo. There is both a passive toggle and a free action to do this. While enabled, your melee attack actions will be able to hit targets near the Echo from any distance.

The action enables attacking from range temporarily and disables melee attacks that aren't allowed from the Echo's space, to make it less confusing which attacks work from range and which would make the Knight run to the target. The toggle passive does not disable unsupported attacks as it's made to potentially be left on permanently.

You can still attack targets around the Knight while this feature is enabled, but you won't automatically move to the target if even slightly out of range, which can be annoying. Clicking on the ground to move works.

Attacks with push/pull effects will be based on the Knight's position rather than the Echo's, which is not correct.

* Opportunity Attacks
Opportunity Attacks triggered around the Echo are made from the Knight's position as when using Attack Near Echo.

Sentinel: One of the benefits of the Sentinel feat is that it makes a creature hit by your opportunity attack unable to move for the rest of the turn. This also works for opportunity attacks you make when a creature moves away from your Echo, and is highly recommended.

War Caster: The War Caster feat allows replacing your opportunity attack caused by a creature's movement with the casting of a spell, and there's nothing excluding those triggered by movement away from the Echo. This is supported by the 2014 version of the feat from War Caster (2014 and UA2).

Unleash Incarnation

At 3rd level, you can heighten your echo's fury. Whenever you take the Attack action, you can make one additional melee attack from the echo's position.

You can use this feature a number of times equal to your Constitution modifier (a minimum of once). You regain all expended uses when you finish a long rest.

Works as written, with some added flexibility. You get to make up to one additional melee attack for each time you took the attack action (regular + Haste + Action Surge) in the turn, but the additional attacks may all be made together at the end of the turn instead of having to be spaced out with the multiple attack actions.

You have to activate a free action to unlock the additional attacks, because I don't like how BG3's implementation of the similar War Priest feature uses up my daily resources without notice.

Echo Avatar

Starting at 7th level, you can temporarily transfer your consciousness to your echo. As an action, you can see through your echo's eyes and hear through its ears. During this time, you are deafened and blinded. You can sustain this effect for up to 10 minutes, and you can end it at any time (requires no action). While your echo is being used in this way, it can be up to 1,000 feet away from you without being destroyed.

Echo Avatar is both somewhat redundant in BG3 since you can fly the camera around, and impossible to implement well since NPCs aren't set up with sensible reactions for encountering something like an Echo. It's not included in this mod. Instead, the next two features are moved one slot earlier.

Shadow Martyr

Starting at 10th level*, you can make your echo throw itself in front of an attack directed at another creature that you can see. Before* the attack roll is made, you can use your reaction to teleport the echo to an unoccupied space within 5 feet of the targeted creature. The attack roll that triggered the reaction is instead made against your echo.

Once you use this feature, you can't use it again until you finish a short or long rest.

Shadow Martyr is moved to replace Echo Avatar at level 7, unless the RAW Level Progression optional file is used.

The reaction happens after the attack roll as that's how this kind of feature needs to be implemented in BG3.

It would have been possible to make it work as written by jumping through some hoops, but getting a reaction popup for every single attack until you've spent the cooldown would get old real fast, so I think this change is necessary for the feature to make sense in BG3.

Technically the attack counts as a critical miss against the original target rather than being redirected to the Echo, which has implications for the Sentinel feat and the Riposte manoeuvre. The attack roll is tested against the Echo's AC and either destroys or misses it accordingly. If no valid position is found near the target the Echo will be destroyed regardless of the roll, but it will still intercept the attack.

Reclaim Potential

By 15th level*, you've learned to absorb the fleeting magic of your echo. When an echo of yours is destroyed by taking damage, you can gain a number of temporary hit points equal to 2d6 + your Constitution modifier, provided you don't already have temporary hit points.

You can use this feature a number of times equal to your Constitution modifier (a minimum of once). You regain all expended uses when you finish a long rest.

Reclaim Potential is moved to replace Shadow Martyr at level 10, unless the RAW Level Progression optional file is used.

Implemented by having the Knight cast a dummy spell when Reclaim Potential can be used, which triggers a reaction that gives the actual bonus. If you don't want it to activate automatically you can set the reaction to ask and skip opportunities as desired.

Legion of One

At 18th level, you can use a bonus action to create two echos with your Manifest Echo feature, and these echoes can co-exist. If you try to create a third echo, the previous two echoes are destroyed. Anything you can do from one echo's position can be done from the other's instead.

In addition, when you roll initiative and have no uses of your Unleash Incarnation feature left, you regain one use of that feature.

Out of scope since it's above the level cap, and supporting multiple Echos would make using other features clunkier by having to select a target.


Extras

Reserve Movement
Swapping places with the Echo has a movement speed cost, which can be annoying if you accidentally move slightly too far and lose the opportunity to swap. To avoid this you get a Reserve Movement toggle, which when enabled holds back enough movement for a swap. The reserved movement is released when you either use up the remaining movement or your bonus actions, as swapping also costs a bonus action.

You can simply click to move where you want to go - the Knight will move as far as possible then stop and unlock the rest of the movement. Then either swap or click again to move the remaining distance.


Toggle Helmet
The Echo has a free action to toggle its helmet. The setting is remembered for new Echos from the same Knight.





Compatibility

Changes Fighter level 3, so is incompatible with other mods that do the same unless some method of merging the changes is used, such as Compatibility Framework.

Object Character Support should be placed before (above) Echo Knight in the load order, if used.

Compatible with War Caster (2014 and UA2). Load order between the two mods doesn't matter.

The Spirit Guardians aura is modified to support suspending it while controlling the Echo. This change is shared with and compatible with my UA6 Invoke Duplicity mod. Load order between the two mods doesn't matter.

This mod patches the target conditions of hundreds of attack actions, including ones added by mods. This makes attacks from other mods support use from the Echo's space without them having to even be aware of this mod.

Patching is done dynamically on startup via Script Extender and shouldn't cause any conflicts with other mods as long as they're properly written. I can't guarantee this won't break things if a spell has syntax errors. Unfortunately BG3 is quite lenient in accepting garbage input, and not all mods are properly written.


Known Issues

Sometimes an Echo summoned in combat will not trigger Opportunity Attacks until its first turn has started. (If its portrait is grey in the turn order.)

High Ground bonuses apply when the Knight makes melee attacks from a distance using the Echo.

Ordering the Echo to move will sometimes cost less movement than it should, only representing a straight-line distance rather than the path actually taken. The game is inconsistent in if the movement cost is deducted or not when the move is ordered. When it's not, I use the minimal possible distance.

The Echo will not work correctly if you change the Knight's body type using Appearance Edit Enhanced.

Game bug: Reaction tooltips often get stuck showing outdated information. This mod has contextual error messages that try to tell you exactly why actions and reactions are not available, but because of this bug the information displayed may be incorrect. Save then load the game to refresh the UI if necessary.

Game issue: Every time you summon a temporary character - Echo or otherwise - it leaves behind a lot of database entries in the save game, which gradually increases load times. This is especially noticeable for Echo Knights since they can summon so frequently. The Database Cleaner mod can be used to clean up most of the remains.


Uninstalling

The main Echo Knight mod can be uninstalled after respecing out of the subclass.

War Caster support can be uninstalled as long as the Echo Knight mod is still present.
To uninstall both; remove the War Caster optional file, load your save, long rest, save the game, remove the main mod.


Credits

Huge thanks to Norbyte! This mod would not be possible at all without Script Extender.
Thanks to Zerd for Concentration Failsafe, which inspired the spell patching this mod relies on.