Gemnemo

Gemnemo (ゲムネモ, Gemunemo) is an intelligent, conscious terrorist microbe that became the first bacterium to master sorcerology.

Plot
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.

When Riel refuses to continue causing more destruction Gemnemo attempts to control him, but is stopped by Li Gado & co.