This guy slaps some grease on the wheels of the plot and keeps it rolling.
Mott is a disheveled dude—he's maybe homeless—who comes up to Terry and Edie in the park. He tells Edie:
MOTT: Your brother was a saint.
Terry knows that Mott knows he was involved in Joey's death. So he's eager to get him out of there, especially when Mott says:
MOTT: You remember, Terry. You were there the night he was—
Terry grabs him and tells him to scram, which he does. But Mott has still played an important role. He's given Edie a strong clue that Terry knows something about her brother's death, which gives her a motive to keep hanging out with Terry.