|Mass Effect 2|
Commander Shepard encounters Mouse while searching for Thane's son. He is described as a former "duct rat", Citadel slang for poor kids who grew up on the station without parents and moved around by using the ventilation systems. Thane mentions that Mouse and some other kids used to gather information on his targets when he was active.
He is the lead on Elias Kelham, the man who hired Kolyat to assassinate the turian politician, Joram Talid. In some outcomes of Kelham's interrogation, Shepard can disclose that Mouse was the one who snitched. Mouse's fate is unknown if Shepard makes this choice, though his future prospects would likely be grim.