Kithlan is a Redguard noble who resides in the Shivering Isles and is Syl's Steward. In the quest "Ritual of Dementia," he can be convinced to give the key to the Private Gardens and Syl's room to the Hero.
He can be tortured or convinced to tell the Hero of Kvatch that Anya Herrick had been acting strange lately.
Kithlan can be told to imprison Brithaur if the main questline for the Shivering Isles has been completed or the Hero is the Duke/Duchess of Dementia.
To learn more about Syl's routine, the Hero should begin by asking around New Sheoth about Syl to determine the best way to get close enough to kill her.