A friend for life
Sometimes, we invest so much energy in keeping our imaginary best friend alive in our fantasies that one day it comes to life. Naturalists who have studied this phenomenon have reached a more stark and less flattering conclusion: it appears to be a kind of unstable symbiosis with a particular species of mimic, attracted in its larval stage by the mental force of the humanoid in question. It is indeed a "kind" of symbiosis because some cases have been reported where the roles of caller and summoned are reversed.
In any case, the result is that, almost every time, the caller sees an inanimate object that captures their thoughts and, in their eyes, best represents their imaginary friend, and then begins to come to life. For better or worse, it must be acknowledged that the targeted humanoids very often possess undeniable artistic skills. Sometimes downright creepy, but undeniable nonetheless.

Master of my Puppet
The Bards of the Ventriloquists' College don't come from any particular school. They just happen to have an animated friend with them. Well, one infested with a mimic, but most would deny it. However, this friend brings them many advantages. And like any good symbiosis, the mimic expects something in return: to be loved, to be fed, and perhaps to take its master's place if he dies.

RP consideration
It is therefore recommended for this subclass to have anticipated, in terms of RP, the arrival into life of the puppet that you will benefit from from level 3. In this case it would be good if the object pre-exists at level 3 (cut puppet, sewn, a simple box with a face drawn on it, an articulated skeleton, a sock with buttons for eyes, … in short, beware of triggers but be inventive).

Level 3

Your puppet is "infested" by a mimic, drawn by the intention you've been putting into seeing your imaginary friend materialize and come to life. This object begins to take on a life of its own. Be aware that this grants you bonuses, but since it's a symbiotic relationship and the mimic has its own will, it comes with minor drawbacks (mostly role-playing issues, but not exclusively).
Your mimic can communicate with you telepathically.

The mimic has the following characteristics: Hit Points: 5 + 5 times the bard's level; Own AC: 1 + mastery level; Passive Perception: 10 + mastery level; STR: 7 (-2); DEX: 14 (+2); CON: 10 (0); INT: 5 (-3); WIS: 12 (+1); CHA: 14 (+2). It has mastered the Charisma save. From now on, it is possible to modify the puppet's appearance using various tools to alter its AC, and possibly magical effects to modify its hit points. At the GM's discretion.

Symbiosis Stade 1
Your puppet appears alive and autonomous when you hold it. You no longer need to think about or control its movements, but you must hold it with one hand. For a creature with more than two arms (trih-kreen, etc.), you must use one of your main arms. Under these conditions, the puppet/mimicry provides the following benefits.

I speak better than you do. If you expend a bardic inspiration when using a spell with a verbal component, whose effect lasts a maximum of 10 minutes, and which involves a saving throw by your target, it is the mimic that emits the incantation, and the target must make the throw with disadvantage.

Let me neogicate. Your mimicry skills are very effective at imitating voices. You gain mastery of the Deception and Persuasion skills. If you already possessed one or both, you gain expertise in one of them. This only applies while you have your puppet with you.

Protect me you fool. Your puppet can take damage. It can also become offended. When a melee attack hits you while you are holding your puppet, it takes a quarter of the damage (rounded down) and the rest is applied to the character.

If it reaches 0 HP: The puppet's mimicry immediately drains your life force. You take 1d4 damage, which it immediately recovers as HP. You must then make a Wisdom saving throw equal to half the damage taken, minimum 10, maximum 20, or suffer the Frightened effect, targeting your last attacker, for 1 round.
Healing your puppet: You can spend 1 hour repairing your puppet with the appropriate tools, then roll 1 die of Bardic Inspiration to restore the result to the puppet. The Mending spell allows you to do this in 10 minutes.

What if the character dies first? In that case, make the death saving throws normally. After three failed saves, the essence of the mimic, along with its remaining hit points, takes possession of your body. See the relevant chapter. If both die at the same time, make death saving throws normally.

Level 6

Symbiosis stage 2
The puppet can now wield a simple weapon with the Light Weapon property. It acts on your turn as a second-hand weapon at the cost of a bonus action. The Two-Weapon Fighting feat applies in this case. It must always be carried and wielded by one of your main hands. This is still the triggering condition for the following effects.

I speak better than you – stage 2. Spending a bardic inspiration allows you to activate this effect for 1 minute for any matching spell.

Push further my Allies. When you grant a bardic inspiration to an ally, the puppet encourages them in its own way. The target can reroll their inspiration die and keep the second result.

Symbiotic mimetism. When you are under the effect of the disguise or deceptive appearance spell, your deception and persuasion rolls are made to advantage, your mimicry perfectly imitating voices, even those never heard before.

Defensive mimetism. You can, through an action, allow the mimic to envelop you for 1 minute, forming an exoskeleton of itself around you. You can then add your puppet's Charisma bonus to your AC.

Protect me better you fool. At this stage of your symbiosis, mimicry begins to take on more importance and to fear a little more for its life.

If it reaches 0 HP: The puppet's mimicry immediately drains your life force. You take 1d4 damage, which it immediately recovers as HP. You must then make a Wisdom saving throw equal to half the damage taken, minimum 10, maximum 20, or suffer the Frightened effect, targeting your last attacker, for 1 round. Furthermore, it insults you during this time, imposing disadvantage on any magical or non-magical attack rolls you make during that round.
Healing: Starting at 6th level, the puppet can be healed with magical healing spells, but with the minimum roll of the relevant die.

Are you listening ?
However, when placed on the ground without you having any contact with her, she can now talk, move her head and arms, but cannot yet walk on her own.

Level 14

Symbiosis stage 3
The puppet is autonomous on the battlefield. It can only wield a light weapon. If it carries a shield, it is necessarily small and only grants a +1 bonus to AC. It acts on its own initiative. It draws spell slots from you and can cast spells from its position. Thus, by consuming your reaction, on its turn, it can cast a spell limited to spells that require a saving throw from the target, with both verbal and touch components.

I'm the only one working here. Once per day, the mimic/puppet can concentrate a spell you have cast. Therefore, it is the mimic that must make Constitution saving throws and does not control this saving throw.

I speak better than you – stage 3. Twice a day, this skill does not consume a Bardic Inspiration die.

You're annoying me, I'm going for a walk
The puppet can move, speak, and act autonomously. However, it cannot do so more than 2x [proficiency level] meters away from you. Its movement range then becomes 6 meters.

The Puppet Becomes Master

If the character fails all three of their death saving throws while the puppet still has hit points, the souls are exchanged.

The character's soul and spirit enter the puppet. The spirit of the mimic inhabits the character, whose hit points remain at -1, and the character inside the puppet is thus left with 1 hit point.
After a long rest, things return to normal. If this exchange occurs three times, it becomes permanent.