Tezuka is undoubtedly the most powerful non-violent player in NPOT, in the sense that his tennis is actually focused around trying to score without injuring the opponent. Since Tezuka has been banished to Germany, who is the most powerful non-violent player in NPOT now? By non-violent I define it as in non violent in action, not intention. We know Oni is a very nice guy, but if you get hit by a ball he's hitting while in Kishin form you'll probably die. Likewise Ryoma has been serving ball in people's face and Kintarou's ability to set nets on fire automatically disqualify them from the non-violent group. Ryoga is out too because he threw an orange at Akutsu's face.
I'd say Sanada ought to be the strongest person in this group though it's hard to see his brand of tennis remaining non-violent after picking up an evil sounding ability known as the Black Aura. You can argue even when he used it it could be in a violent way (shredded Yukimura's headband, and who knows what he could've hit if it he had more control). Leaving Sanada as unknown (since he really hasn't played since losing to Yukimura), and ignoring any characters who have shown nothing so far, I'll go for a tie between Irie and Atobe. Irie obviously was a lot stronger than Atobe, but Atobe has improved, so I give them a tie for now. Fuji is likely to challenge this spot when Konomi remembers that he exists.
How is Kimijima not mention at all? He have seriously the most hax skill given by Konomi in the series. Negotiation can seriously get anything done. Was there any hints at all before that Yukimura was not heal completely until it was reveal via negotiation? Konomi can seriously make him win against any characters in the series if he wants to. Yes, even Echizen/Tezuka/Sanada. What if Nanjirou/those two grandpa had a health conditions somehow that needs doctors from America to heal? And that's just an example. What about dark negotiation? Let me win or mafia comes after your family. There's a lot of potential with this tech.
There's things more important to people life than Tennis. Negotiation is the uber hax skill with potential to triumph anyone in the series that Konomi wants Kimijima to win against. Of course, we haven't really Kimijima with his limiter off yet, but well, I'll repeat it again, Negotiation is the most uber hax skill in the series.
The only characters I can see being immune to negotiation is Atobe. Seriously.