She is dual class thief/mage.
This means that at level 4 she stopped being a thief and became a mage. Now all her exp. go to her mage class, she will never advance as a thief.
Jan is a better thief (multi-class thief/mage, so will advance in both classes), but Nalia is a really good mage so worth having if you need a good aligned mage.
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Time is an illusion. Lunchtime doubly so.
Epona of The Laughing Hyenas