For Keldeo since she can talk with telepathy she would have heard from other humans (who are jealous of MC) the MC capturing other legends but would have misunderstood and think the MC been making slaves of the legends and would fight him to free the legends if they meet and would fit with the character of seeking justice. Once she has been added to the harem a side scene could be with Mew where Keldeo turns into her resolute form and can control the shape of the horn to not be sharp but bigger, longer and bumps where the MC has an idea to spit roast mew in middle and mew would use her psychic power turn herself around while dangling off the ground and taking both at same time. Mew would definitely love it and Keldeo would like the pleasure from using her horn.
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Keldeo is telepathic?! (quickly googles Keldeo) ah, in the movies.
I think I'd slightly adjust your plan so that Keldeo is trying to free the other Swords of Justice once they've been caught, maybe I can write in something to keep them busy so they can't just correct her.
I can work on the idea.
Keldeo growing her horn into a dildo, now that's something I can have fun with!
I'll probably make it so she can grow it out of her crotch and use it like a strap-on, Mew would definitely enjoy being sandwiched between two heroes!