in (one's) corner
Giving one's full support to someone. I'm nervous about the trial, but I'm glad I have you in my corner. I feel a little bad that he's going into the meeting with no one in his corner.
in someone's corner
on someone's side; giving someone support. This idiom and fight your corner are boxing metaphors and refer to the diagonally opposite corners taken by opponents in a boxing match. Trainers and assistants are in a boxer's corner to offer support and encouragement between rounds.