Aia's party briefly included Tiax, the deranged gnome illusionist that can be found quite late in the game, namely in Baldur's Gate. I had quite some laughs while he was in the party, especially when he got into banters with other NPCs. He's one of those characters that's actually impossible to get on with, unless one is on the same level of madness. All discussions with him end with his quote "Tiax rules all!".
His dialogue has been slightly enhanced through the BG1NPC Project but it's been done in a fitting and humorous way.
Tiax, however, doesn't tolerate good-aligned parties well, so he left the party after only a day or two. It's been for the first time that any of my parties even allowed Tiax to join them. He's one of those characters that are encountered so late in the game that by then most parties are probably already skilled enough and not in a need of another member.