Monster Cat

Left alone, Monster Cat is nothing but a small cat, light brown with darker brown striping.

Once the Quirk fired under Trigger and it merged with a bus, the animal's face manifested below the front window, a second tail joined the first, and the drug turned its tongue black.

Under normal circumstances the cat behaves like any cat, simple and docile. Trigger flipped that into violence and erratic rage, and yet the animal underneath never fully vanished: Knuckleduster redirected it with an improvised cat toy, and it later chased Koichi Haimawari purely because the antennae on his All Might hoodie looked worth pouncing on.

The cat boarded a bus in Tokyo while Koichi Haimawari was sightseeing with his mother Shoko and his friend Makoto Tsukauchi. One of Kuin Hachisuka's bees injected it with Trigger, which fired and amplified its Quirk, fusing the animal with the vehicle and sending a sentient bus swerving through the streets. Knuckleduster wrote the incident off at first, until he spotted Koichi aboard; he taped a feather duster from Pop☆Step into a lure and drew the thing onto the expressway, away from a packed shopping district.

Ingenium arrived and boarded to size up the danger, keeping quiet about knowing Koichi. Team Idaten pulled up with a truck and Enigma used Gigantification to move passengers across, Koichi and Shoko among them, but Kuin dosed the cat again from a distance and it went berserk, wrecking the truck and knocking the driver and tour guide out of the bus. Ingenium caught them, leaving Makoto alone on board with a ruined overpass ahead.

Running low on fuel, Ingenium needed help, so Koichi went back out as The Crawler with grapefruit juice to restart the Engine Quirk. He drew the creature's attention, it mistook his hoodie antennae for a toy and pounced, and Ingenium burned Recipro Burst to intercept, overheating himself in the process. The Crawler chased the bus over the edge, unmasked, and got aboard for Makoto. In freefall he forced a new trick out of Slide and Glide, a momentary push through open air that carried him to Ingenium, who took Makoto the rest of the way down. Everyone survived, and Kuin walked off with the cat, which had come through the drop intact. Koichi could only manage a 30 centimeter hover when he tried to repeat the move later, calling it a double jump and suspecting it was a one-off born of adrenaline.

Its Quirk, also called Monster Cat (化け猫, Bake Neko), lets the animal leap into an object of its choosing and fuse with it, taking on that object's properties and a hybrid shape. The bus is the only recorded use, and because Trigger was involved throughout, how the power behaves normally remains unknown.