Thinking of Kakuzu when he was tasked with assassinating Hashirama and failed spectacularly.
Hashirama being himself, grinning at this very serious guy offering to give him another go once he gets better at fighting. Kakuzu outraged and going at him mindless to be put back on his ass immediately.
So Kakuzu takes him up on that offer and the next time they come to blows it’s over as soon as the first time. And each time Hashirama gives him pointers on how to improve because he can see the potential and what a waste it would be to not hone it, shape it and free it into the world of shinobi.
Their meetings become more training exercises than assassination attempts but not in the way Hashirama and Madara meet out in the battlefield, holding back and never reaching an accord. Harsh words of dead brothers and never ending angst.
No, these two could feel the thrill between their sparring, the heated dancing of their chakra when jutsus were spat out. It was intoxicating and liberating despite the nature of their meetings.
Like his younger brother, Tobirama, Kakuzu had a knack for experimenting and developing new techniques. And gods above was it a sight to behold, the day Kakuzu had revealed his newest technique and the explanation of the second heart his body now possessed.
Hashirama wanted to poses this warrior. He had realized then.
Have him on the playing board to fight for him and only for him.