Walahnan is a powerful chronomancer and a man who suffers from germophobia. But worry not, he can prove to be more than useful. While his ambitions do not overwhelm those around him, he certainly has some aspirations that inspire his studies. The mod does not include a romance, however, you may get an option or two to show him you like him throughout Shadows of Amn and Throne of Bhaal. To give Walahnan the chance to join you, complete Nalia's quest and check the rest of De'Arnise Hold area. He should be near the rocks to the east of the keep.
To keep Walahnan in your team, you'll need to complete his personal quest.