[Probably just Medusa works best for ultra-tsundere! As does them being a couple or at least friends beforehand. Combination of all of the above? She's obviously hornier than usual and making excuses and doing it because she likes him and his big dick but is going to complain about it the entire time. Maybe she wears something provocative, invades his bathtime, or aggressively sits in his lap, basically anything where it's obvious she wants to fuck but the moment he pops a boner she's going to blame everything on him. What do you think for setting?
Otherwise, what are you into for kinks? From you post, I think rough sex, marathon and size difference can obviously apply. And of course big dick, the bigger, the better, and excessive cum. Any particular positions/acts? Given Medusa being an absolute little monster about this, the weirder ones likely to come up would be footjob, prostate massage (possibly via her tail), using her powers to give him perma-boner, and just anal sex in general (being all, "ugh you pervert!" about it or "this is all you deserve!" about it).]
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Otherwise, what are you into for kinks? From you post, I think rough sex, marathon and size difference can obviously apply. And of course big dick, the bigger, the better, and excessive cum. Any particular positions/acts? Given Medusa being an absolute little monster about this, the weirder ones likely to come up would be footjob, prostate massage (possibly via her tail), using her powers to give him perma-boner, and just anal sex in general (being all, "ugh you pervert!" about it or "this is all you deserve!" about it).]