A Pupil is a character that has a master, or mentor. They can be used to teach their mentors spells that they would normally be unable to learn.
After creating a character from the "create a character" bill in the dark assembly, that character that pays Mana to create the pupil will become the mentor, and the newly created character will become the pupil.
Also, whenever the pupil Levels up, the mentor may gain a small boost in all stats. However, the mentor only gains the stat boost if the pupil is using the same weapon as the mentor, or the the pupil is the only one the mentor has.