There is no character that is safe from being NTR'd. If she's strong, just subvert her strength or don't use force. Create a situation where she or others can't use their strength, or create a situation where the NTR is willing from the get-go and no one would interfere because it's what she wants.
For HSDK, just have like, a Chad ntrman step in when Kenichi is trying to help Miu and save him (though he's actually just trying to impress the babe), and then get dragged back to the Dojo to become a disciple with Kenichi. Then just have him slide in and seduce Miu out from under Kenichi gradually. Could have him specialize in weapons training with Shigure to slowly seduce her too.
If you want something a little more openly ntr, just have like, a magic style rival dojo challenge the dojo to show whose disciple is better through a series of challenges over like, six months, and interfering would forfeit. Then have ntrman disciple curse Miu in various ways to lower her strength, and the only way to get rid of it is to do increasingly erotic stuff. She'd keep it under wraps so no one would come after ntrman and cause a forfeit.
If injecting magic is too big of a stretch, the dojo always seems broke, so have a situation where they cause a bad injury to some rich fucker (make it Kenichi's fault for the juicy irony), and have him threaten to sue. Can't beat him up because he'll just sue more. Then he makes a secret deal with the girls that starts off slow, like working as maids in his mansion while he recovers, and escalate from there.
