Boo almost always appears to greet Strider in the 1-3 second disclaimer at the start of Strider's top lists and at the credits. Although he doesn't ever say anything beyond a chuckle, he appears to be a friendly ghost and a moral support to Strider.
When Strider has previously been attacking things as Dark Strider, Boo has shown sadness and appeared to attempt to stop him. He also led Strider to his new light saber after his old light saber broke.
Boo also joined Strider in a later series called Boo's Clues, where he offers Strider hints to figure out problems or simply what Boo would like to do.