Formerly a duct rat, Mouse was one of the kids who used to give Thane Krios information on his targets. He used to have broken teeth, scabby knees, went around barefoot, and at some point had a mother in his life. In return for his services Thane often rewarded him with treats such as real chocolate.
Mouse begged Thane to take a holophoto of him when the drell made to depart the Citadel for a while. He wanted to have someone remember him by. Many years later, Thane's son Kolyat presented the holo to Mouse as proof of who he was, intending to find some work on the Citadel. Mouse put the word out using Thane's old contacts, and eventually the criminal Elias Kelham offered employment.
Commander Shepard encounters Mouse in the middle of a comm transaction while searching for Thane's son. Mouse is spooked at seeing what he thought were the retired Krios and the dead Shepard showing up in front of him. He is unwilling to give up information on Kolyat's employer, but is eventually coaxed or coerced into doing so.
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.
When asking Captain Bailey about Mouse's crimes he mentions that Mouse illegally sells Shepard VIs. If Shepard asks Mouse about the VIs, it is possible to coerce Mouse into handing one over or demand a cut of the proceeds.