Bowser (Paper Mario Series/Mario and Luigi Series)

"Pfft! Please, who do you think I am? I'm Bowser, the Koopa king!" (Mario and Luigi: Bowser's Inside Story)

Bowser is perhaps the most prolific villain in all of gaming, appearing in hundreds of games as either the main antagonist, a supporting role, or, on rare occasions, the protagonist. In many of his appearances, he simply serves as a beefed up enemy to act as the captor of the princess as well as the final boss. He doesn't say a word and may as well be the dragon from Saint George and the Dragon. But in both of the Mario RPG series', Bowser is full of dialogue, engaging actively in the story and adding more personality than either Mario or Luigi. Within the five Mario and Luigi games, two traditionally turn-based Paper Mario games, and Super Mario RPG, Bowser acts as the main antagonist only twice, otherwise acting as a party member, or a rogue character within the story.

Bowser is almost synonymous with the Super Mario franchise. To have a Mario game that doesn't feature Bowser in some way would feel like an incomplete experience. However, there having been 8 traditional Mario RPGs at this point, having Bowser be the main antagonist of each game would have gotten stale by the third game. In each game, he is written in as an unpredictable element who can derail the story, kidnapping Peach in the midst of an alien invasion in Mario and Luigi: Partners in Time, or crashing the final boss of Paper Mario: The Thousand-Year Door.

In each of his appearances he reflects the "gimmick" of the game inflicted onto Mario. He is imbued with dreamy power in Mario and Luigi: Dream Team Bros., paired with his younger self in Partners in Time, and united with his paper counterpart in Paper Jam.  He represents a concerted effort by the writers to implement an important piece of Mario's world into his more expanded stories, and shows that there is a point that a character can reach where they are as crucial an element to the series as the main character.