Nalia de'Arnise is a Chaotic good female human thief/mage who is recruitable early in the game. She can be found at the Copper Coronet establishment within the Slums District. If approached, she attempts to hire the player's party to rid her family's castle of monsters which have recently invaded. By her description it seems that the de'Arnise stronghold might be infested with trolls and Yuan-ti. But luckily Nalia knows of a secret passage to enter the estate because a frontal assault is not possible.
Since Imoen, the game's central thief/mage, is kidnapped early on in the game, Nalia makes a suitable replacement. As a human, she dual-classed early on from mage (level 4) to thief. So while she can pick locks she won't be able to outclass a specialized thief player or a character like Yoshimo.
The de'Arnise estate is ultimately the Fighter's Stronghold which can be obtained by a Fighter, Monk, or Barbarian. Should the player complete Nalia's quest and cleanse the castle of its evils they will become the lord of the de'Arnise estate and will be given the opportunity to govern over it's lands in a few dialogue driven quests about subjects such as: theft within the guard's barracks, flooding in the neighboring farms, and deciding who a maid should marry.