Guide by Nikolai from PN Forum
For masters, you can have an apprentice up to 3 days. During these days, your apprentice accumulates EXP automatically and once the three days are over, you will receive the EXP. Some players might also challenge you and steal your apprentice. But you gain the EXP they have accumulated during the time they were your apprentice. You get mail notifications when someone tries to steal your apprentice.
Each day, you can 'Teach' an apprentice. This will give you a fixed amount of EXP. You can do this for each apprentice per day.
Benefit: You can use your apprentices' character in Valhalla (even though they are not online). However, to use your apprentice in the stage, your apprentice cannot be 11 levels higher than the stage level. For example, Endless Sea Forest is for level 26+, so your apprentice can be level 27, 28, 29 ... 36. If the apprentice is level 37, the system will not allow your team to enter.