1) Boris's voice actors, Bob Hoskins in the first film and Charles Fleischer in the sequels, both starred together in Disney's and Touchstone's film Who Framed Roger Rabbit as Eddie Valiant and the voice of Roger Rabbit, respectively. 
2) Boris, Balto, Jenna & Muk & Luk are the only characters that have appeared in all 3 movies. 
3) In the closed captions for Balto II: Wolf Quest, it says that Boris has a Yiddish accent. As in Balto he has a Russian accent. 
When Boris' describes himself, his last name is Goosinov. This would make him the only character with a confirmed last name.