Let me try to write up a more or less full walkthrough:
Basic:
You want to have all skills and attributes, so just grind the normal mode a few times, fuckign her as often as you can for maximum points.
Gameplay:
The game is a little complex, I will try to explain the main mechanics:
Her "Gauge-Bar" is the little thing under her HP bar that fills up after you hit her, or she attacked. It empties slower the lower HP she has. If you use a special attack while she has some Gauge bar, the special attacks have special effects.
Your special Bar: Under your HP bar, there is a smaller bar which fills when you hit, get hit, or block attacks. It is your special bar, and you need it to perform special attacks.
The main Battle menu:
Top left: Opens the special Attack menu, remember you need special bar and her to have Gauge Bar, to have those attacks to special effects.
Middle left: Block. Consumes two action points, reduces incoming damage and fills your special bar.
Bottom left: Counter Fuck. Have her below 1/4 HP and use it to counter her kick attack to unlock an H scene.
Top-Right: Upper attack, deals most damage, doesnt fill much of her gauge bar.
Middle-Right: Middle attack, deals medium damage and gauge.
Bottom-Right: Low attack. Deals the least damage, but fills the most gauge bar.
The Special Menu:
(all these effects only occur, if you have special and she has gauge bar!)
Top left: If she has gauge, these attack will strip her.
Top right: If she has gauge, this attack will know her down on all fours. You can then Doggy fuck her to end the round.
Bottom left: If she has gauge, this will chain her up. You can then fuck her from behind or have her forced cow girl style to end the round.
Bottom right: This will knock her out. You now have 3 H options: Top-left will force a BJ. This one does not end the round. Bottom left will fuck her missionary style while she is fainted. Bottom right is Doggy style.
Game Modes:
Extramode 1 is unlocked by beating and not raping her in the normal mode. The dialogue choices in normal mode dont matter. If you rape her in the Extramode1 (or she you), this mode will change to Extramode2, so no more good endings.
Extramode 2 is unlocked by beating and raping her in the normal mode.
Extramode 3 is unlocked with the Co-Fallen ending. It is basically just an endless mode against the possesed girl.
Extramode 4 is unlocked with the love ending. It is an endless mode against the normal girl.
Endings:
Love-Ending: Start Extramode1. Beat her 3 times. No H at all. For dialogue choices select the frist one after the first KO, and the second one after the 2nd KO.
Co-Fallen: Beat her two times. Select the first dialogue choice after the first KO, and the second one after the second. Then lose the third round (but dont get hit by her H moves!)
Rape-Ending: You just beat her 3 times while fucking her. I dont think dialogue matters.
Lewd-Ending: You beat her 3 times, but make sure she cums at least 3 times. Which means you can just beat her down, fuck her at the end of each round and let her cum each time.
Now a list of all "ingame" H scenes
1.) Normal Fuck: Just chose the right option after winning a round.
2.) Counter Fuck: Get her below 1/4 HP, then wait for her to do that Kick attack. Use the bottom left option right when she attacks.
3.) Doggy: Knock her down using the top-left special attack
4.) Chained Cowgirl: Use the bottom-left special attack, then use the left H move.
5.) Chained From Behind: Use the bottom-left special attack, then use the right H move.
6.) Forced BJ: Use the bottom right special attack. Then select the upper left H move.
7.) Fainted Missionary: Use the bottom right special attack. Then select the lower left H move.
8.) Fainted Doggy: Use the bottom right special attack. Then select the lower right H move.
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The following moves occur when you fight against her demon form and get hit by her tackles:
9.) Face Sitting.
10) Blowjobs, Handjobs and titfucks.
11.) Cow Girl.
12.) Foot job