Sometime prior to Uprising, Grimian was hired as an agent of Krytus.
When Grimian first appeared, he single-handedly defeated the Battle Force 5 and retrieved the Battle Key. When Captain Kalus orders that Grimian hand him the key, he refused to and challenged Kalus to Kio Hakoko. He won the trials, but rather than follow tradition and kill Kalus, he instead spares his life and has him spend the rest of his life in shame.
Kalus later upgrades his vehicle and beats him again (after an interruption by the Battle Force 5), but like before, he spares Grimian's life and appoints him second in command, stating that, "A wise one once taught me, keep your friends close and your enemies closer". His alignment with the Red Sentients is revealed afterwards when he reports back to him, away from everyone else.
When Grimian's alliance to Krytus is revealed in Grimian's Secret, Kalus punishes him by permanent banishment, with execution upon return.
He however returns once again with Krytus' help along with a faction of Vandals in Rumble in the Jungle. However, he is ultimately defeated after the final defeat of the Vandals. He's then last seen with Kalus approaching him with a spear in his hand, indecating he was killed for his treason.
Grimian's vehicle was the Prime Evil, modeled after an oversized quadrunner. It has a nitro booster for cranked-up speed and limbs to batter enemies to submission.