Collis is Rosemary’s friend, and he’s essentially following her around. He’s there to witness Dick’s big break-down, and helps Baby when she’s trying to get Dick out of jail. More importantly he tells Dick a story about Rosemary with a young guy on a train that makes Dick fall for her even harder. Rosemary thinks he’s a "bumpkin." And he’s also a groupie/stalker, following Rosemary wherever she goes.
It’s not entirely clear whether Collis is simply a device to provide information, and help the plot move smoothly, or whether he is something deeper. Maybe he’s the truly wonderful stand-up guy who truly loves Rosemary, but she’s just too blind to notice him. What do you think? Is Collis actually important to the novel, or could he be removed with no big difference. Did it have to be Collis who tells Dick the story about Rosemary? He sure gets lots of lines and airplay. Maybe he’s more important to the novel than is apparent at first glance.