Alp

Encyclopedia Entry:
A peculiar kind of Succubus unlike any other kind of monster. They were originally Incubi who suddenly transformed into monsters. Normally, a human man can never become a monster. As of the present, there are no males among the monsters at all. When it comes down to it, an Incubus is just a human warped by Mamono mana. Due to the existence of mana, which human men continually produce, no matter how strong the Mamono mana that's imbued in their body, human men remain human.

Alps are Incubi that deep withing their heart strongly desired to become a woman, or be with a man, and used the Mamono mana taken in upon becoming an Incubus, which is normally used to boost their mana generation faculty, to instead destroy it. As a result, They're unable to produce mana, and they lose all their mana. Since they are left with only Mamono mana stored in their body, they change into a monster. Even after becoming Alps, their hearts, values, memories, and personalities, everything other than their bodies, is still the same from when they were men. Their body has completely changed into that of a woman, yet they self identify as men. They may not even be consciously aware of the desire to become a woman which changed them into an Alp, or they may have been in denial, with the truth hidden away deep within their heart.

Thus they are shocked by the radical physical change, and there's a huge gap between it and their male hearts, which likely confuses them. Most of them would probably say "Having sex with a man like a monster is disgusting. Impossible. I hate this." However, once the monster's instinct begins to take root in their heart and body, they'll naturally be drawn to human men. The monster's instinct within them makes a rapid change, becoming bigger. The feeling of dissatisfaction due to not having obtained any mana will become unbearable, and they'll begin to strongly desire a human man. Eventually, they'll surely end up losing to the monster's instinct.

Once that happens, in a horribly confused state due to the gap between disgust and desire, they'll assault a man and have sex with him. When Alps first taste the pleasure that women and monsters can obtain, which is so intense it's incomparable to what a man can experience, they may almost lose consciousness while shaking their hips without knowing what's going on. When the man's mana is released, and they experience ecstasy, they become enthralled by their first taste of female pleasure, and the taste of mana, which will be sweeter than anything they had ever eaten until then. They'll likely still be confused by the feelings in their own heart: "The man before my eyes wants more.", "I wanna have sex more." Then, when the heart of the man mixes with the pleasure of a monster inside their head, while spending days in want, they'll end up attacking and having sex with a man again. While repeating this, eventually they'll lose all resistance to having sex and making love with men while still completely retaining their own self-awareness as a man even though they used to say they hated it so much. On the contrary, they'll come to seek sex on their own and enjoy the taste of pleasure and mana.

They were friends with other men before becoming an Alp. Even after having sex over and over and completely falling in love with their human male partner, normally, they'll still treat him as a male friend, no different from how it was back in the old days. And then, they'll start to tempt and have sex with the man as if it were a matter of course with the same feeling as inviting another male friend to hang out.

Additionally, there are also Alps that awaken to the joy of being a woman through this process. In that case, the body changes, rapidly growing voluptuous to invite men, and the heart changes to be like that of a woman, resulting in an Alp that's even more feminine that a real woman, and even more lewd and lustful than a Succubus.

Trivia

 * The Alp is the only Mamono to originate from a (living) human male.