You need to fuse a Jack Frost using any two personas that will result in a Magician-arcana persona. The average levels of the personas must be between 5 and 8.
Then you need to get Jack Frost enrolled in medical school. It's difficult, given that you have a very small window of time to actually complete the game, and medical school is a long and complicated process (usually taking several years), but don't give up. Talk to Mitsuru; perhaps she can get him a room at the dorm.