For the question about defilement and the growth of character.
First - MC won't level up per each battle, battling random tentacles in map is only for gold/satisfaction seeing the scenes.
Second - Her corruption is slowly but also surely, while u advance story, a certain shop will offer a process to amplify her stats(that will trigger her corruption at the cost of 1k) and another shop that sells good weapons and gear(every time u get back, he will use an aphrodisiac on her saying he is preparing medicine). Events triggered on dungeon that affects her such as random tentacle from the ground or wall, or walking on them also amplifies corruption. It all depends on the weak spot u choose as u start the game(ends up getting double corruption from those).
Third - Right as the game start she is raped and implanted with an Auraune Seed. This is a flower demon half-woman, half-plant. Since this is a hentai game, they added that semen is the "water" that drives it, so the more actions she does, the more her transformation into an full fledge auraune will happens.
And people asked how to get the good ending?
Avoid corruption, don't let her stage with the flower advance(every time it does, she gets stronger and a window pops up saying she is more corrupted and can do more actions such as prostitution later on, or even changes in some minor events from earlier). Basically since random enemies gives no exp, u gotta level up doing good deeds(such as cleaning the graves, kicking the bandits outside of stuff and so on, or protecting the girl being raped in the tents... those are samples on how to level up without affecting ur corruption a lot). Basically if u trigger corruption to max, her gain daily(yeah, she naturally gains defilement daily) will double and in less than 10 days will trigger the bad ending already. For a "good ending" gotta avoid every corruption possibility in the game while doing good deeds to level her up and also remember to keep using the "purify the seed" option in the church daily. Once u reach close to the end of the dungeon, will change to another mc(black haired girl) and it means u're in the way of the good ending or so.
A point that really bothered me is... that the corruption gauge isn't visible anywhere in game but every action adds points in the value, whenever it reach/bypass certain amount, the flower will evolve triggering boost in her stats. Whenever it reachs a certain amount, u can sleep and get a bad ending the very other day. Maybe an save editor would be able to see and change the variable freely to avoid game ending asap when watching the events around though.