Fenrir is Atreus's wolf.
Upon first meeting the Rangers, Fenrir was aggressive towards the young heroes, mostly due to their own mythical beasts. However, he was calmed by Atreus and he allowed Raiden to pet him. Raiden was endeared by the wolf who seemed to enjoy the lombax petting him. He licks her and shows his now more docile and adoring side to his personality. Over the series, Fenrir and Ice Blister develop a strong bond and friendship with each other as they are seen getting excited upon seeing each other and sometimes playing tug of war with a bone.
Fenrir also becomes close to Raiden as he likes sniffing and nuzzling her arm.
Trivia:
Fenrir is the only pet of an Allstar who isn't an OC.