Bowser
From Paper Wiki
Paper Mario
Bowser, aided by Kammy Koopa, second in command of the Koopa Troop, took control of the Princess's castle by lifting it into the air with the power of the Star Rod in Paper Mario. Mario, along with his friends Goombario, Kooper, Bombette, Parakarry, Bow, Watt, Sushie and Lakilester, had to rescue the Seven Star Spirits in order to use the Star Beam, which would deactivate the Star Rod. After the initial fight against Bowser, he retreated to the roof of his castle. Using the Power Platform Kammy Koopa built, Kammy boosted his size and power to ensue his victory. The Star Beam failed to work against Bowser. However, Princess Peach discovered that by wishing for Mario's strength to increase, and Twink granted her wish by giving Mario the Peach Beam.
Now that Mario was able to negate the Star Rod's effects, he had no problem taking Bowser down and returning everything to the way it once was.
Paper Mario was significant because it revealed another of Bowser's motives for kidnapping Peach; he has a crush on her and hopes to steal her away from Mario.
Paper Mario: The Thousand Year Door
Bowser is one of the antagonists of the story, although not the main one. He appears many times in the story, such as chapter 3, when Mario is fighting, Bowser suddenly comes in and challenges Mario to a duel. His second encounter with Mario is chapter 8, right after Mario is to fight Sir Grodus, the secondary antagonist of the story.
Super Paper Mario
Bowser Is the most powerful partner in Super Paper Mario He is able to breathe fire, a far more powerful attack than other attacks, and his jumps deal double the damage. Playable from Chapter 3 onwards except from two points; from after Chapter 6 to the middle of Chapter 7-2, and from the end of Chapter 8-1 to the middle of the fight with Count Bleck