It was one of the ghosts caught by Professer E Gadd who turned it into a painting, along with many other ghosts of the mansion. It was freed with all the other ghosts when King Boo attacked E Gadd's lab., Boolossus resides on the balcony on the third floor of the mansion.
Boolossus is at first a massive and slow Boo that tries to crush Luigi by bouncing on him. Luigi must force Boolossus to touch the horn of one of the two unicorn ice sculptures to break it into the smaller Boos. Luigi then has to use spray them with ice to freeze them and suck them up. After a while, the Boos will form Booslossus again. It grows smaller and faster as more Boos are deteated. At last, the last Boo is sucked up and Boolossus is once again returned to its painting.
- Boolossus is a combination of the name "Boo" and the word "colossus".
- Boolossus appears in the Mario UNO cards on the "Draw 4" card.
- The name "Boolossus" also literally translates to "Giant Boo".
- In the PAL Hidden Mansion, Luigi rides the Poltergust like a car during the Boolossus battle. This provides faster transportation, but also less traction.
- Boolossus is the only Portrait Ghost to not have HP.
- Along with King Boo, Boolossus is one of the two only Boos that are Portrait Ghosts.
- Oddly, Boolossus is Ghost #22 in the gallery even though he is the 16th ghost caught.
- Boolossus has other two names, "Jumbo Boo" (junbo teresa in Japanese) and "Buulussus" (in German and later translated to English).