Thank you very much for playing!! I'm glad you enjoyed! Hmm, while I do know that a lot of players enjoy self-insert protags when playing VNs, Nelli isn't one, hence why her name/appearance is not changeable (and customizable appearance would unfortunately be difficult to implement for a game with as many animated illustrations as this). I do have a couple games planned with self-insert protags without defined appearances, though! I hope you enjoy them :)
=== SPOILERS ===
Haha, I'll go into the transformation lore a little bit more in the prequel/sequel (which are both in scripting phases)! Nelli had no idea about Raku's true self because he kept it from her, and he basically hasn't transformed much in the past four years while he was at university. He doesn't need to transform, and he can eat normal food for calories (although he does have a liking for raw meat). He can control his transformation (only transforming partially, or transforming fully - and he can also control the size). He's part of a family of "creatures" like him, and it's genetic. The world is inhabited by different species of creatures like him, but they mostly stay on the down-low, hidden from human society.