got this question in strawpage and it genuinely left me thinking for a while
its essentially impossible to come up with stuff for these two without leaving speculation to go straight into headcanon territory, but it's especially hard to me to think what would make Celebi appealing to a specific character, because she gets so little to do in the game...
My best bet would be that, much like with grovyle, he would also admire celebi's strenght, resilience and unwavering will? I think she could light up his days the same way she does grovyle's. I do wish we had anything more than celebi calling him a "coward" in a very specific context. You can barely argue that adjective has any deeper meaning when dusknoir's plan is literally hide behind grovyle's corpse to surprise hero and partner, the "coward" feels too apt for the situation to go much deeper.
(We dont even get to see why Grovyle likes her in the game, we see them after they've become inseparable, and thats only really apparent in se5, bc in the main story grovyle just says shes a bit weird and celebi teases him sometimes. i wish we had more clues of how they behaved in regular environments)
tbh my fav thing for them is having them separated and only coming together because of grovyle. they both revolve around him, so they find common ground in it, and their love for him is what pushes them to open up to each other. it's what makes the most sense to me considering that dusknoir does not even verbally acknowledge her in the main story. id say itd be suspicious if it wasnt bc of 2000s misogynistic writing.