Bowser is Mario's rival. In very few games Bowser helps Mario and Luigi. The reason Bowser despises Mario is because Bowser is in love with Princess Peach but Peach doesn't feel the same way. Because of this, he kidnaps Peach many times and Mario saves her. In Super Mario Galaxy, Bowser tried to make his own universe and tried to make Peach his queen. Luckily, Mario destroyed his plan.

big bowser