Hello, from my multiple tries I can give you a quick and rough ending walkthrough, the game gives you a big illusion of a choice, but really most things don't matter until chapter 5. The boy always gets corrupted no matter what (but there's a resistance path at the end), the girl wether you corrupted her or not, you can basically reverse it either way in chapter 5, making me wonder what was the point of this corruption counter at all...
So basically choice from chapter 1-4 don't matter for the endings, except for one event at the end of chapter 4 which can lock you out of one ending but doesn't affect the others, so you better do it. The event is: when you get hypnotized by the witch, don't resist and you will have sex with the incubus.
Otherwise, from chapter 1 to 4 do what you like, I encourage you to do all the lewd paths. There's a few little variations here and there though, for example in chapter 2 if you did the lewd things before and solve the puzzle, then you will have an extra lewd scene compared to if you failed and gave up on the puzzle (if you didn't do lewd before and solve the puzzle then you get no extra lewd of course). Other variations I can think of revolve mostly around the fact that if you opt out of some lewd scenes, you are given the choice to skip others later, instead of being forced into them, so not really worth mentioning.
So the ending walkthrough... Besides that single event in chapter 4, it mostly starts branching on chapter 5, when you're controling the boy in the prison, after reading the new rules, this is when you want to make a save.
- First ending: full corruption: as the boy, accept to suck the succubus tits, that's pretty much it because doing that forces the rest of the choices on you. You get an alternative chapter 6 where both the girl and boy "willingly" surrender to the demons, the girl is on her way to transform into a succubus.
- Second ending: the boy's betrayal: as the boy, refuse to suck the succubus tits, but then have sex with her. You get an alternative chapter 6 where the girl finds the boy in the library, and he has become very familiar with the succubus to say the least. At the end he joins forces with the demons and they defeat the girl.
From then on, for the other endings, you need to access the "regular" chapter 6, which also have several branching paths depending on a few choices. For this, as the boy, refuse to suck the tits and refuse to have sex with the succubus. Then starting chapter 6, the boy and the girl will have sex. If you did the final event I mentioned earlier on chapter 4, then you will have the option to think of the incubus while climaxing, you need to do it too if you don't want that one ending locked out. Then you have to go through a lot of dialogue until the boy and the girl are trapped in two separate corridors, this is where it will start branching again (I'll only mention the choices that matter)
- Third ending: the boy's betrayal 2: as the boy, give in to the succubus dance, and as the girl, tell the incubus you prefer having sex with your boyfriend if prompted (only happens if you fantasized about him during sex with the boy, otherwise the girl automatically traps him which doesn't matter for this ending). From this point, the ending is pretty much the same as the second one.
- Fourth ending: the heroes' failure: as the boy, endure the succubus' dance. Proceed to the end fight. During the fight, as the girl, succumb to the incubus Tempting gaze. The girl surrenders and become a happy sex slave.
- Fifth ending: fell in love with the devil: This is the end that is dependent on the scene at the end of chapter 4, so quick recap: you need to give in the hypno sex scene, go through chapter 5 with the boy resisting both the tit suck and the sex with the succubus, and then when the boy and the girl have sex at the start of chapter 6, you need to fantasize about the incubus while climaxing (won't appear if you didn't do the previous event).
When in the corridor, have the boy give in to the succubus dance. As the girl, tell the incubus you fantasized about him while having sex with the boy (won't appear if you didn't do it, obivously). Then, join the incubus in the bed on the side. The girl becomes one of the incubus many lovers.
- Sixth ending: true ending: as the boy, endure the succubus' dance. Proceed to the end fight. During the fight, as the boy, resist temptation from the succubus and as the girl, resist the incubus Tempting gaze. The girl defeats the incubus and the couple is able to escape, resolved to train and become true adventurers, and find the true descendant of the goddess (a sequel maybe?).
- Seventh ending: truer ending? (pimp boy): Follow the same step as the true ending, except during the fight, have the boy be tempted by the succubus (either pink choice is fine). The ending will be the same, but after the credits, you are given the choice to see the secret epilogue, so choose it. You see the boy and the girl having sex in the girl's house, and making plans for the future. You then control the boy, go near the church to find the succubus and have sex with her too.
And that's a wrap.