The MC's ability is gluttony which allows him to obtain the ability of whatever he eats. It has the side effect of healing his wounds, increasing his stats, and making him h**ny. The first thing he eats is a slime which allows him to freely change his body into a slime and absorb corpses and excrement.

Mizuki's ability requires having s*x with the person they love to unlock and become decipherable. When it's unlocked it's found that Mizuki multiplies the power of herself and her lovers the more they love each other. She develops bis*xual tendencies so she's boosting the MC as well as his harem. This multiplier multiplies their stats and adds additional effects to their skills.

Mizuki is really the gatekeeper and manager of his harem. Her ability also allows her to control the pregnancy outcome of their activities.

The country that summoned them is in a dystopian decline due to the ruling of the incompetent tyrannical king. The MC's class was summoned to be used as throw away soldiers to invade the thriving neighboring country and claim their resources.