Aster is the last of his race, the pookas, who were completely destroyed by Pitch and his Fearlings. From the books we come to know that he came to distrust humans, because of what Pitch did, and that he was proven again and again that humans have their dark side, and that lot of times they use it.
He is quite old, he is older than the Earth, and in the book there is a passage, where he tells Katherine about the fact that he had to rearrange the shape of the Earth, because it was egg-shaped and that meant it wouldn't be able to exist, so he dug various tunnels and changed it in circular, whit the materials that remained he made Himalayas and Australia.
When North and Katherine find him in the second novel of the series, he lives
alone in his egg-shaped warren along with his warrior eggs, and is not really
welcoming to them. His biggest obsessions are chocolate and eggs, which drives
North nuts, while Katherine uses it for their purposes and actually convinces
Bunnymund to help them, which leads to the fact that they become friends.