Kooper

Kooper is one of Mario's many sidekicks in the game Paper Mario. He comes from Koopa Villiage and decides to join Mario's party after Mario helps him reclaim his shell from a fuzzy. He is cold-blooded, ruthlessly rational, and dangerous.

Character Development
Kooper first joins Mario's party in Chapter 1, in Koopa Village. He speaks in a deadpan neutral voice, and doesn't seem to understand compassion. He places a high value on logic, often pointing out Mario's inefficiencies. Kooper is shown to be dangerous early on, and his appetite for murder only grows throughout the game. After leveling up to super rank, Kooper gains the ability to kill on command. Kooper now only experiences enthusiasm when he is gaining more power. At the end of the game, Kooper is the one who delivers the final killing blow on Bowser, which may have been a mistake, because he now believes himself to be all-powerful.

Mario
Mario is probably the only person in the world that Kooper "likes" and really the only person that he could be said to have a relationship with, mostly because of his profound sense of loyalty and submission to Mario as a result of his admiration of Mario's power.

Goombario
Goombario, much like the rest of the party, isn't liked very much by Kooper. Unlike the rest of the party however, Kooper doesn't just merely dislike him but has deep and utter loathing for the Goomba, and would've murdered Goombario already if it wasn't for Kooper's deference to Mario.

Bombette
Bombette and Kooper don't interact too much, but Kooper does have some form of respect for her.

Trivia

 * The roots of Kooper's characterization come from ProZD and Manky's dislike of Kooper as a character. They find him (as well as Goombario) to be a a bland and boring partner, especially compared to his counterpart in the sequel, Koops. Because of this, they tried to make him sound as bland and boring as possible. However, this ultimately evolved into the much more interesting angle of portraying him as an emotionless killing machine.
 * Kooper is Asexual.