From the Super Mario Wiki
“Hey, Mario! Since you didn't want that cake, I ate it. It was delicious. The most delicious I've ever eaten. Strawberries, frosting, the works. Man, you don't know what you were missing.”
Shellshock is a minor character from the game Paper Mario: The Thousand-Year Door. He is a common Shady Koopa who became a contracted fighter in the Glitz Pit, the martial arts arena of the floating town of Glitzville. Shellshock and his fighting team, the Shellshockers, are ranked #7 in the fighting roster and thus are a part of the major-league.
Mario encounters Shellshock in the midst of chapter three, while he is searching for the Gold Star. After Mario defeated the Armored Harriers, Mario will switch to the major-league and he and Shellshock will become roommates. If Mario manages to rise his rank up to #8, the player will have the opportunity to battle Shellshock's fighting team, and if he wins, his ranking will go up further.
At some other points in chapter three, Mario will receive two cakes that were apparently sent by a fan. Mario can eat the first one, which will replenish his stats. If he decides to eat the second one however, Mario's partner will be disabled by poison. The player can also choose to ignore the cakes, which will result in Shellshock eating them while Mario is away fighting. He will mock Mario how he enjoyed his meal after ignoring the first cake. When Mario returns from his match against Chomp Country after having declined the second cake, he will find Shellshock in a disabled state, lying on the ground and squirming, probably from a stomach ache. He will recover from this after Mario ended another match though.
A figure who is apparently Shellshock appears in the cutscene during the final fight against the Shadow Queen, alongside other athletes. There he can be seen cheering Mario on through the Gold Star. It is, however, very likely that this character was originally supposed to be KP Pete and his sprite was simply misplaced with that of a Shady Koopa; said character uses terms in his speech that were used exclusively by King K throughout the game (like "G-man").
Shellshock appears just like a regular Shady Koopa, meaning that he looks like a Koopa Troopa with a notably darker skin tone and a light blue shell. He wears handcuffs and a neckband, both covered with spikes, and a pair of sunglasses. The shoes he wears are of a similar blue as his shell is.
Not very much is revealed about Shellshock's personality. He seems to have slight confidence-issues, since after Mario defeats him in battle, he begins talking about how he loses motivation when he sees how Mario is progressing. In addition, Goombella mentions that he appears like a big softie. Another side of Shellshock's personality is that he appears to be very fond of sweet things, since he cannot resist to eat Mario's cake without even asking him. Like many other characters throughout the game, Shellshock has a characteristic way of speaking, often ending his sentences with the word "baby".