Is the Echo of a Echo Knight actually a creature?

Question

I've always been certain that the echo created by an Echo Knight fighter using their 3rd-level feature Manifest Echo is not defined in game terms as a creature, since the description of the echo never explicitly tells it is a creature. But the presence of people believing the echo to be a creature in lieu of the fact that it is immune to all conditions and might make saving throws (game characteristics typically associated with creatures only) brought this doubt to my mind.

This tweet from Jeremy Crawford implies it is just an image and not a creature, further confirming that the feature should tell you, but this other tweet implies that the echo is an object whilst the feature doesn't explicitly say so.
(Included for reference and insight, since tweets are not eligible as official ruling)

Is the echo a creature?

Answer 1

The echo is not a creature.

The echo is defined as an image, and nowhere in its description as a creature or object if not only by some degreee of inference. If the echo was a creature, the feature manifesting it (creating / summoning) would explicitly tell you that game detail, as most other already existing effects do.

There are many effects, specifically illusion spells and similar ones, capable of creating an image which can look like a creature, yet there's nobody denying that that image is just that. There are illusions that can have an AC whilst being just illusions, like those of the spell Mirror Image, disappear when taking damage, like the ones of the spell Project Image, or act as a proxy for the caster to perceive / take actions through it.

To name a few spells that create / summon creatures and go out of their way just to tell you that this new thing is, in fact, a creature (emphasis mine):

• Animate Objects:

Choose up to ten nonmagical objects within range that are not being worn or carried. [...] Each target animates and becomes a creature under your control until the spell ends or until reduced to 0 hit points.

• Simulacrum:

You shape an illusory duplicate of one beast or humanoid that is within range for the entire casting time of the spell. The duplicate is a creature, partially real and formed from ice or snow, and it can take actions and otherwise be affected as a normal creature.

• Other spells which provide a stat block (something unique to creatures) and still define the newly created / summoned thing as a creature are Animate Dead and Create Undead, all the Conjure spells (e.g. Conjure Celestial or Conjure Minor Elementals), and all the other summoning spells (be the demon/devil summoning from Xanathar's Guide to Everything or the Summon spells from Tasha's Cauldron of Everything). The same logic applies to the special companions gained by Warsmith Artificers or Wildfire Druids (or animated by a Creation Bard) via their class features.

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To clarify, there are also spells and features that create objects (or force stuff) that can have some similarities to creatures, yet those still include a line to define this new thing as an object (or force). Some of those spells are Bigby's Hand and Unseed Servant, or the Eldritch Cannons created by an Artillerist Artificer.
If the second tweet marks the intended function of the feature and the echo should behave like an object, it can be ruled as such until an errata of Explorer's Guide to Wildemount or a sage advice about it comes out and gives us stronger confirmation. But reading the feature as it is worded now, it is not an object (or at least it isn't clearly one).

If a game effect does not define the new thing as a creature or an object, it is neither. Mage Hand, for example, creates a spectral floating hand, and the spell Mordenkainen's Faithful Hound conjures a phantom whatchdog that is just a spell effect and not a creature.

Answer 2

It acts like a creature in certain ways.

There's no formal definition of "creature". There are, instead, many properties typical of creatures:

• A creature type
• Ability scores
• Skill bonuses
• Hit points
• Armor class
• Saving throw bonuses (and the ability to make saving throws)
• Turns (and the ability to take actions)
• Size class (and the ability to control space)
• Movement speed (and the ability to move)
• Equipment
• Resistances, immunities, and vulnerabilities

Of these, the echo explicitly has:

• Armor class
• Hit points
• Saving throws
• Immunities
• Movement
• Size class / controlled space

And pretty clearly does not have:

• A turn. Instead, it uses the fighter's turn and they can take certain actions through it. This is the strongest argument for not treating it as a creature. (On the other hand, getting a turn is not an absolute! Controlled mounts, for example, have to share someone else's turn.)

Now, objects often have hit points and armor class. They can be described with a size class but they don't control space. They don't have saving throws or the ability to move. In a crazy coincidence, the two questions we've had lately that touch on the echo's status as a creature involve (1) whether it has to make saving throws against area effects, and (2) whether it provokes opportunity attacks when it moves. That is, the uncertainty is exactly about ways in which it's inadequate to treat the echo as an object.

I think this reveals a correct intuition: the echo can be treated like an object, except when it can't. This approach fits the 5e theme of describing generally how things work and then trusting the players to figure out edge cases.