In SMT:SJ the demons do not learn skills as they level up. Each of them come with at most three learned skills. When the analysis gauge of a partner demon is full, it will present you a demon source during the next level up. The source will contain at most three fixed skill and one random bonus skill. The random bonus skill will be drawn from currently what skills it knows. If you are not happy with the random skill, you may reset and get another one, it will change.
At any time the player can only have one source from any one demon. Once the source is used, the demon may give you another one during leveling up, but the chance is much lower.