Hel Wolf is a demon from Hel, that was captured by the Asgardian Gods to be used as a weapon against The Serpent and his army.
Major Story Arcs
The Gods of Asgard has captured the Hel Wolf and caged him in a mystical cell. Their plan is to let him lose on the Serpent's army, but Loki makes a deal with him: the Hel Wolf's freedom for his binding servitude to Loki as long as he lives. The Hel Wolf tries to go back on his promise but Loki uses a magical bridle forged originally for Thor's goats Toothgnasher and Toothgrinder, to force the Hel Wolf to submit. Loki then hides Hel Wolf in the World Tree to keep his location from Heimdall's sight.
When Loki returns for him the two next go to Hel to seek an audience with Hela, but Garm blocks the entrance, so Loki releases the Hel Wolf on the guard dog. The two fight and, unknown to Loki till later, they mate. When Loki returns he rides Hel Wolf to Mephisto's realm. When Loki returns to Hel Wolf, they then go to Midgard where Hel Wolf is present when Mephisto and Hela make a pact between themselves and Loki. After they leave Loki betrays Hel Wolf and has the Disir kill him so Loki can ride his soul to Limbo. In Limbo, Hel Wolf tells Loki their deal was only valid as long as Hel Wolf was alive and now that Loki had him killed he was free to eat Loki. Loki escapes but Hel Wolf is eaten by Surtur. Meanwhile on Midgard Tyr gives Hel Wolf a funereal prayer he feels the Hound doesn't deserve.
When Loki rode Hel Wolf into Hell and left him to fight Garm, the two fought for a while and then mated. Sometime later Garm had a litter of 7 puppies, that she sent to Loki care of for Hela. One of these puppies is Thori, who became Loki's pet. Hel Wolf then later finds Loki in a nightmare and blames him for his death, and shows what death is like inside Surtur.