There are many gods, heroes and monsters in Norse mythology - far too many for me describe here - nor is there need as there are many fine sources on the web. Wikipedia alone has a multitude of excellent entries based on the primary sources. The Poetic Edda and Prose Edda in particular are vital sources, but many other texts contribute to our knowledge of ancient Germanic mythology. With that in mind I will concentrate on the characters most important to Harkan's story.
Angrbotha is a female giant (jötunn). There is little mention of her except that she is the mother of Fenrir, Hel and Jörmungandr.
Fenrir is a huge wolf and a son of the god Loki. A prophesy states that he will kill Odin in the final battle at the end of the world (Ragnarök).
Loki is a mischievous god who does more ill than good. He is a shapeshifter and takes many forms in the tales. He fathers a variety of other norse characters. With Angrbotha he fathers Fenrir, Hel and Jörmungandr. On one occasion he even acts as a mare and gives birth to Odin's eight-legged horse, Sleipnir.