One could be clearly Boruto Uzumaki. Maybe all the Karma’s are related to one another and that why Kawaki was able to activate and use Boruto’s Karma ability when he was in need. That’s what could be the first and foremost possibility. But later Kawaki says judging by the way Boruto was talking about Karma, Boruto hasn’t mastered it yet.
That’s why I think that it was his own Karma’s power and not Boruto’s. So, who was Kawaki referring to when he said that it’s this guys power? In my opinion, he was talking about the Otsutsuki that he has killed and whose power he can use through Karma seal. As there have been many theories suggesting that if the user’s seal went out of control then the Otsutsuki that resides in the seal can be reborn and thus killing the user in the process. I’ve come up with an interesting theory that in Naruto series we had Jinchuurikis who possessed huge chakra and strength and if tailed beasts inside them got control over their body, tailed beasts would become free.