Kovalev helped Kovalev. He lost he had his reasons of overtraining which given his track record I don't fully agree with. I think he was underprepared. He realised he had to listen to his coaches or he'd lose again. He was always better than Alvarez but at times in the first fight and even at times in the rematch Kovalev didn't look so right. But, Buddy and Cruz did all that they could. Trained Kovalev for a 12 round fight. Kovalev paced himself for a 12 round fight. Did anyone here think after say round 4 that Kovalev would win by knockout? I never saw Kovalev winning by knockout without taking a risk of being knocked out. I think Kovalev will try to go for 12 rounds again not going for the knockout. it might be harder vs Bivol, Beterbiev and Gvosdyk to do the same, that's not to say he can't beat all 3. Kovalev is no longer the Krusher. he's at the victory lap level I don't know who he fights next.
He is a (strength and) conditioning coach. He also instructed Kovalev what to eat and what not to eat. How to control his weight in a smart way, etc.
From what I understand he is Buddy's longtime partner. They trained Arturo Gatti together, for example. What is his background (before that) I don't know.
There was a point in one of the later rounds Alvarez looked like he might go. But I think Alvarez going 12 had everything to do with his durability, not Kovalev who will still "Krush" 95% of LHW opponents.