It's Mr Pants

Development

Mr. Pants replaced Donkey Kong when Rareware left the Nintendo fold during the games original production. Titled Coconut Crackers, the Kong title also had a more 3D appearance to its graphics. When Microsoft picked up the company, and THQ the distribution rights to all Rare GBA titles, Mr. Pants took over, and the 3D design was replaced with a more friendly 2D interface.

Protagonist

Mr. Pants was the mascot of Rare's original website and previously had cameos in Jet Force Gemini and Banjo-Tooie. In Britain, pants means underpants which is why Mr. Pants is just wearing underpants.

Information also contributed by Kartanym