Leela is a spy and the daughter of Osman, head spymaster of Al Kharid. She can be found just north of The Skulls Headquarters (the old jail) in Draynor Village and features heavily in Stolen Hearts, Diamond in the Rough and Missing My Mummy. It is implied in Missing My Mummy that Osman and Leela are descendants of Pharaoh Senliten. Only one of her descendants or someone working for them can avoid the curse of her tomb.
When Senliten is fully restored and Dealing with Scabaras is completed it is possible to travel with Leela to Senliten and have a talk with them. This is a requirement to start Do No Evil.
According to Ozan, he met Leela when they were younger, starting a series of events that lead to his exile from Al-Kharid.
There is currently a glitch, where if you took Leela to Senliten's tomb before some of the new quests, such as Diamond in the Rough, in the quest log, it will not count towards the requirements for Do No Evil.