Riel

A beyondian with an extremely frail and thin body. Riel is a friend of Leo, and both get beat up and mugged a lot.

Riel is convinced by Gemnemo to undergo his procedure to become stronger. After becoming a giant because of it and seeing the chaos he caused and how lonely he was, he regrets accepting Gemnemo’s offers. Gemnemo tries to take over Riel but is stopped by Li Gado with Sonic and Leo’s help, while Riel gets administered something to reverse the gigantification.

He ends up becoming really tiny, but is happy with the outcome.

(Source: Blood Blockade Battlefront - Kekkai Sensen Wiki)