Leif is a denizen of the realm known as the Pinnacles of Ice. When you first free him, which you can do at Level 37, he does not remember who he is. Many of the quests he gives you are designed to help him discover his identity and backstory, which you eventually learn all about when Lorelei gives you the quest series designed to walk you through building his stained glass window in the Hall of Heroes.
As mentioned, Leif also gives us many quests. Below is a list of all the quests he gives, in order. If you'd like to see a list of quests given by all characters, please visit the main Quests page.