If he doesn't have any costume changes, you might as well model it as one object.  Clothing animations have to be coordinated with the character's other movements, the same way you would animate long hair on a character.  Flexible things hanging off a character, whether hair or cloth or rope/chains/a tail/whatever, typically move like a sine wave, a ripple moving from the part attached to the character to the part swinging free.  The simplest thing to do is have you are a friend wear a longcoat and videotape this person doing all the actions you want your character to do in the game so you can capture how the cloth moves.

If he doesn't have any costume changes, you might as well model it as one object.  Clothing animations have to be coordinated with the character's other movements, the same way you would animate long hair on a character.  Flexible things hanging off a character, whether hair or cloth or rope/chains/whatever, typically move like a sine wave, a ripple moving from the part attached to the character to the part swinging free.  The simplest thing to do is have you are a friend wear a longcoat and videotape this person doing all the actions you want your character to do in the game so you can capture how the cloth moves.