Roy's feelings about Mullet Fingers change as the book progresses, so I can't state a single feeling. I can do it in two general feelings though.
The first feeling that Roy has about Mullet Fingers is curiosity. The first time that Roy sees Mullet Fingers is from the bus. Roy is being bullied by Dana Matherson, and Roy sees Mullet Fingers running shoeless next to the bus. Roy's head is immediately filled with a bunch of questions. Who is that kid? Why no shoes? Roy vows to find answers to his questions. His curiosity only grows when he sees Mullet Fingers again and is told by Beatrice Leep to leave the kid alone.
Roy eventually learns who Mullet Fingers is and why he is living in the woods. Roy also learns that it is Mullet Fingers who is causing all of the construction problems in order to protect the owls. With all of his questions answered, Roy's feelings toward Mullet Fingers change from curiosity to friendship and respect. Roy even joins Mullet Fingers's crusade to save the owls.