I Was an Undying, Yet Somehow Became a Golden Knight

The story might overlap with @Relative Amnesia Next Door, but the plots are different. The roommates are all nonhuman creatures: Insects, Orphnochs, and Fangires. They transform into different Riders, though. In human form, they become Kabuto, Sasword, Delta, and Blade. In monster form, they become Great Sword, Dark Kiva, Dark Kabuto, and the Emperor of Earth.

Nan Gongshuo was a chubby otaku and a closeted romantic, but he unexpectedly transmigrated into the world of Blade, becoming the King of Spades, a South Asian Hercules beetle Undead. After the Battle Fight, he wound up in a new world where everyone could awaken as a Rider. Upon arriving, Nan Gongshuo awakened as the Golden Rider and acquired the transformation devices of several villainous Riders. He was assigned to Dormitory 777, where all his roommates were monsters, not a single one human. Once they learned each other's identities, they began playing a rather 'fun' game.