The Mystique Main Character has the following Level Up Power:
Level Up (10) - Whenever Mystique gains ATK, she gains that much XP.
The rules have this to say about +1/+1 counters:
Many effects put +1/+1 of -1/-1 counters on characters. These numbers alter a character's ATK and DEF until something removes the counters.
Does adding a +1/+1 counter to Mystique cause her to "gain attack," which triggers her Level Up Power?