A partner is a character who accompanies the player character in investigation phases of the Ace Attorney games. Originally an unofficial and colloquial concept used to refer to a character who accompanied Phoenix Wright and Apollo Justice in their investigations and trials, the release of Ace Attorney Investigations: Miles Edgeworth explicitly refers to "partners".
In the core series, the player can usually talk to the partner directly at the Wright Anything Agency / Wright & Co. Law Offices. Generally, the topics of conversation in these situations are "What to do" and "Any ideas"; that is, the player can ask the partner for hints on how to start the investigation. The partner also usually travels alongside the player character (and thus is not a "resident" of any one accessible location), and can engage in the player character's conversations with other people or talk about an object being examined.
In Ace Attorney Investigations: Miles Edgeworth and Gyakuten Kenji 2, the partner is more clearly identified. The partner's sprite follows Miles Edgeworth's or Gregory Edgeworth's sprite as the player moves them around. The player can press "Y" or the "partner" button on the touchscreen to converse with the partner. At one point during The Imprisoned Turnabout, Little Thief occupies the "partner" button, and it is used to switch between two different re-creations of the prison courtyard at different times.