Companion AI

Companion AI describes the behavior model driving an NPC that accompanies the player through combat and traversal, and its whole reputation as a design problem comes from how visibly it fails: getting stuck in a doorway, aggroing an enemy the player was trying to sneak past, blocking a narrow corridor at the exact moment a quick retreat was needed. A companion that fails this way doesn't read as a flawed teammate — it reads as a bug wearing a character model, because the player has no lever to correct it beyond hoping the next patch fixes the pathing.

The genre-defining trick most well-regarded companions use is a fictional cheat: Ellie in The Last of Us is scripted to never take damage and never draw enemy aggro during stealth sections, which is not remotely how a real second person moving through that space would behave, but it's invisible in play precisely because it removes the failure mode players actually hate. The one place this cheat is off the table is an Escort Mission, which needs its companion killable for the objective to exist at all — a structure that inherits every companion-AI failure mode at full price, which is most of why it's so widely hated.

Companion AI is best understood as a special case of Game AI generally, but an asymmetric one: most game AI is built to present a fair, competent challenge, while companion AI is built to look competent without ever actually winning or losing on its own — its job is to seem like a teammate, not to play well in any sense that would be measured independently. That asymmetry is also why companion AI ages differently than enemy AI: an enemy that plays dumb is forgiven as an easy fight, but a companion that plays dumb breaks the fiction that anyone else is really there.