Kasumi actually died when she fell off the cliff. They took Shizuki (her twin sister without glasses) and she became the next host after Alice died. It went haywire. When her wantings was to bring Kasumi back. She was brought back by Shizuki's dream. That's the Kasumi that was actually in the cryogenic chamber with the other 159 members. The girl who was in the sleeping beauty bed made of thorns was Kasumi's dead body. The Kasumi that was placed in the cryogetic sleep gained all memories of the other. Giving it new ones of easily going into the room with everyone else by themselves. She doesn't have medusa, since she was recreated with a dream. And the little boy doesn't now either since his dream was used in the whole project (the vines and monsters from his video game). It was explained in the movie that if your dream comes to life, you'll be medusa free. Like Alice and her Monkey Alien looking thing. Therefore, neither of them have medusa.