In the Paper Mario series, Bowser usually appears as a villain, although not always the main one.
In Paper Mario, Bowser is the main antagonist, having stolen the Star Rod and kidnapped Princess Peach, with help from Kammy Koopa. In the end, he is defeated by Mario. This game is significant in the Mario series, in that, it reveals that Bowser wants to steal Peach away from Mario, since he has a crush on her.
Paper Mario: The Thousand-Year Door features Bowser as a playable character, although he is also a minor villain. He is on a mission to find and kidnap Princess Peach, who has been taken by someone else, as well as hunt down the Crystal Stars. He appears as a playable character in some cutscenes, some of which are side-scrolling levels similar to those found in Super Mario Bros.
Bowser's appearance in Super Paper Mario is similar to that in Super Mario RPG, where he is an enemy at first, but eventually becomes a party member. In Paper Mario: Sticker Star, Bowser once again becomes the main antagonist, where he invades Decalburg during the Sticker Fest and splits the Sticker Comet into pieces. He ends up getting one of the Royal Stickers and defeats Mario in combat, kidnapping Peach in the end.
