The Chimp is very competitive and challenges Mario to challenges where Mario has to beat The Chimps record in something (ex, chimps stomp challenge beat his record score in a time limit). Rather than forcing Mario to face him he invites him to challenges through letters, although when you meet him he acts as if he was expecting you (example: "It's about time you got here!"). He loves being a rival to others and he also has a very big ego.
You first meet him in the Fluffy Buff Galaxy. You have to find the Toad Brigade Captain in this level. At the end it appears the chimp challenged him to a race but he lost and got really weak and tired and gave up. You go to the chimp and talk to him he says "You may be a worthy high climbing rival, here's a star for your troubles.". He then gives you the star of the level.
The Chimp does not give out any green stars, but his challenges are often hidden until the player gets a letter from him.