Loki

Loki was the son of a giant (or jotunn. We say giant in Swedish, but the word is a bit misleading since many of them are of the same size as the gods), but he once mixed blood with Odin and became his foster brother (Odin's father was also a giant, btw). His role in the Norse mythology has been debated a great deal by scholars, among other things how much he may have been influenced by the Christian Devil (since those who wrote down the Norse myths were Christians). He is a bit of a trickster, sometimes assisting the gods and sometimes causing problems for them. He is described as cunning, unreliable and disrespectful, but there are many times when the gods were benefited by his tricks. It was thanks to him that Thor got his hammer, for instance.