The problem of hooking up with a weak monster...

"That was....we definitely need to do that again sometime." Pulling her little body off the bottom of the sink, she continues to clean herself up as the Pixie hums a cheerful tune. Asuka seems more or less indifferent from what she's seen of you two. "That's great and all that you are two found a way to fuck. But you know, when you fuse her you are just going to lose your little fucktoy here?"

"Uhh...he's not gonna fuse me, because I am awesome at jerking him off." The Pixie doesn't look at Asuka as she continues to clean herself. Asuka does have a point. Pixies are cute but they don't last long against serious monsters. Demons can still gain levels, but overtime they will still be outclassed by monsters who are innately more powerful.

There's only about two ways a Pixie can end up surviving long. Either by fusing them into the older and more powerful High Pixie, or by a much more complicated way that you aren't even sure will work. Monsters still inherent memories from their old forms. If they can inherent skills from the fusion material that makes them, they must keep some memory of their old lives. Because of that, giving something that has a grudge against you, a more powerful body, would be exceptionally stupid.

Most demons and angels alike aren't usually unwilling to abandon the idea of becoming stronger and generally even encourage it. There is still just a number which this Pixie seems to be apart of, that isn't willing to let go of the idea of completely abandoning their old self for all the obvious reasons.

The only other way a Pixie can become a serious monster would be in a way that you can't even sure would work in real life. In the third game, there was a labyrinth based in hell and in that maze, there were these things called Demon Doors. The only way to open them was by oddly specific actions. If you managed to open them, the rewards were almost always great.

One such door, required the first monster that you recruited in your game. That just so happened to be a Pixie. If said Pixie was still with you and you managed to get to the demon door, then that Pixie would be upgraded to level 80. Not only the level increase, but with the kind of stats that are objectively better than Beezlebub's. Not to mention some of the best attack and healing spells in the game. Of course, to make the Uber-Pixie, you'd have to have some idea where to even find a real world demon door or one that does that.

It's just a theory. But if the real program is similar to the game....then it is just maybe possible that something like that exists.

You need to think about your options carefully.