Ronnie is a mouse that lives in the Tree Fort. His only known appearance is in "BMO Noire," where he is dating Lorraine, but finds out that she's cheating on him with Bebe. Ronnie gets jealous and kills him by taking out his batteries and frames BMO for it. Ronnie is later killed in a police shoot-out and set up by Lorraine as the patsy for the whole thing.
However, this could all be a part of BMO's imagination, as suggested in "BMO Noire."
The colors of Ronnie are unknown, due to his only appearance being "BMO Noire." He has eyes similar to Jake's and a plump body.