Gemnemo

Gemnemo is an intelligent, conscious terrorist microbe that became the first bacterium to master sorcerology.

He developed a “hyper augmented method for accelerated cytological division” or in simple words, a means of enlarging and strengthening living things. He arrives at Hellsalem's Lot and there convinces Riel to undergo his procedure to become stronger and take revenge on all the people that bullied and hurt him because of his frail body.

(Source: Blood Blockade Battlefront - Kekkai Sensen Wiki)