Brinn is first encountered when the party enters Moonrise Towers, where he remarks on them being a True Soul, and that Disciple Z'rell would want to see them. Talking with Brinn lets him welcome the party properly, and he nudges them towards the audience chamber and throne room, where Z'rell is, mentioning that she will find work for the party.
If the Nightsong is freed and the Harpers launch their assault on Moonrise Towers, Brinn will be part of the remaining forces defending Moonrise Towers, and can be encountered in the ground floor hall standing behind Disciple Z'rell.