"They know what I am. Let them come, if they find the courage." -Fenris
"What has magic touched that it doesn't spoil?" -Fenris
"Mages in glass houses shouldn't throw fireballs." -Fenris
"I do love the way the merchants glare at me, as if I'm going to steal something." -Fenris
Fenris shares his name with the wolf Fenrir of Norse mythology, which is spelled and pronounced Fenris in Scandinavian languages. Fenris is an elvish name given to him by Danarius, which means "Little Wolf."
Fenris primarily swears in Tevinter, but occasionally will break into meditation-like comments on the fight. He has one of the few soundsets that never actually yells during combat.
The name Leto may be a possible reference to Children of Dune (by Frank Herbert). "My skin will not be my own," said by Leto in reference to accepting the sandtrout to change his own skin. In both cases, the appearance of their skin changes and grants them special abilities. It means "by/with Death" in Latin.
Fenris has a particular dislike for fish and states he likes apples.
Fenris apparently plays cards with Varric, as during party banter in Act 3, he comments that Fenris owes him some gold, and claims he'll ask Hawke for the money when Fenris says he'll pay him. Alternatively, if Isabela is still in the party, Varric will express amusement that Fenris believes he can win enough coin from Isabela to repay him.
In Act 3 and Legacy, if neither Isabela nor Fenris have been romanced, then party banter between Fenris and Isabela suggests that they are in an intimate relationship, although it appears to be purely physical.
Fenris, as a former slave, never learned how to read. Dialogue with Hawke can open up the option to teach him how, which can lead to a comment by Hawke when selecting the bookcase at the Hawke estate in Hightown.