Fenris Wolf is the son of Loki (the Norse Aesir God of Mischief) and the giantess Angrboda, the brother of Jomungund (The World Serpent) and Hel (Goddess of the dead), the half-brother of Sleipnir and Samirah al-Abbas and an antagonist in the 2015 Magnus Chase and the Gods of Asgard book "The Sword of Summer". He is based on Fenrir in Norse mythology.
Fenris was the son of Loki and Angrboda and was placed on an island called Lyngvi and was tied down with a rope called Gleipnir. In is said that when Ragnarök arrives, Fenris would be free and would eat the sun and kill Odin (the leader of the Aesir gods).
The Sword of Summer
Fenris was responsible for causing the death of Natalie Chase (the mother of Magnus, a demigod son of Frey) by sending two of his wolves to the apartment in Boston where Magnus and Natalie lived (this was to affect Magnus and make him go to look for Sumarbrander, the sharpest sword in the Nine Worlds). He also killed Blitzen's father Bili (who knew Gleipnir was getting weak) and had convinced Odin to make Samirah al-Abbas (Loki's demigod daughter) a Valkyrie.
When arriving on Lyngvi, Magnus, Samirah, Blitzen and Hearthstone come across Fenris and find him surrounded by the bones of the children of Thor, Odin and Tyr and was calm despite being bound, but saw that his weakness was heather. The group were able to rebind Fenris with a new rope.