// season one
Boris seems to be Kai's replacement upon his leaving the Demolition Boys for the second time.
We aren't told anything about him apart from that he was "specially trained to block out all emotion except anger and hate" by Biovolt and that the scientists training him have all "mysteriously disappeared", with most assuming that Boris most likely killed them. Whilst we have no idea of exactly how he was trained, one can assume those weren't the best years of his life as they have left him extremely wary of trusting anyone except his own team (it seems to fit that he grew up with them). It is said that there were hints in the Japanese anime that Boris had been abused as a child, but these were obviously cut out of the dubbed version.
He isn't really seen in action (except for a brief battle against Max to steal Draciel) until his battle with Rei (Ray) in the second round of the World Championship final, in which he is seen to be nothing short of ruthless, quote "I would say farewell, but that would be lying, since I do hope you fare badly".
His training has allowed him to attack the beyblader as well as the blade, and, despite eventually losing the match, he manages to seriously injure Rei by harnessing the air itself as a weapon. Rei was only able to survive the onslaught due to Driger sacrificing itself to protect him (making it twice that season that Ray lost Driger).
// season two
Whilst Boris doesn't appear at all in this season (along with most season one characters), it's assumed that he is free from the abbey and Borcloff (Boris Balkov) and still with his team, perhaps trying to get accustomed to life outside of Biovolt.
// season three
Season three (and I'm wondering where Ivan went…) finds Boris still suffering from not being able to trust anyone but his team, but he is a lot less violent than before, perhaps suggesting that freedom from Biovolt's grasp has allowed him to relax. He certainly looks a lot better than in season one (or is that just my opinion?) appearing much less like just another one of Biovolt's soldiers.
Still loyal to the team, even though Kai's return (can't he make up his mind already?) has put him on the bench for the Championship, Boris joins Yuriy (Tala) and Sergei (Spencer) in trying to bring Borcloff (Boris Balkov) and the BEGA League down. After managing to get into the building and being defeated by Garland, however, Boris’ air-time is cut short as both he and Sergei fail to reappear at all in the show (unless you're lucky enough to be watching the original version with the special credits at the end).
// my view
As for why I like the guy so much, I guess I just find him very intriguing. We know next-to-nothing of his back story, of the experiences that turned him into the boy that was happy to follow orders to actually kill others, which I suppose leaves people a lot of space in which to come up with their own ideas. He just seems like the sort of person who nobody knows what they’re thinking or feeling, and he probably has no idea himself either. I mean, it must have been hard for him to come-to-terms with emotions in the gap between seasons one and three, and he still finds it difficult to overcome his inability to trust people in general.
To me, he is one of those fascinating characters that have you watching every episode just in case he makes an appearance. The lack of Boris towards the end of G-Revolution was actually irritating, since I then had to put up with Takao’s annoying self.
Anyway, the guy lacks internet love, so I stuck up this fansite-slash-fanlisting for him, in the hope that there was more than just a spattering of Boris fans out there. Turns out that there are.