You need a thief, and Jan is a good thief. Nalia rarely finds any traps or picks any locks, so you need to get either Jan or Yoshimo. Jan talks a lot, and can be fun to have in your group, he is also quite good at picking pockets, find illussions, open locks, and find traps, which is almost all you want from a thief. He can backstab just as good/bad as Yoshimo, but he level up very slowly (like your Aerie) because he is a multiclassed character. It takes twice as long for him to level up compared to single classed characters.
Yoshimo on the other hand can set special traps that you can throw to the place you want instead of going there. But he hasn't many points in set traps, so you need to raise that ability. He doesn't have many points in pick pockets or detect illussions either, but maybe you don't need that.
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