Note that if playing with a team, the player landing the killing blow on the Ancient Dragon unlocks that class's ending for everyone. For example, if you are a Squire playing with a Monk teammate, and your teammate lands the killing blow on the Dragon, both you and your teammate will unlock the Monk ending.
To find out which hero endings you've already seen, go to the 'Accomplishments' section of your statistics window.
From the Depths - Completed all Area 1 levels on any difficulty setting
To the Rooftops - Completed all Area 2 levels on any difficulty setting
A Taste of Victory - Completed all Area 3 levels on any difficulty setting