Notes on game mechanics:
PROTIP: if your attack is equal to their defense, you are considered the winner at no penalty!
For fighting:
Your attack power is (level x 10) + 20 + bar modifier.
"Bar modifier" refers to the little bar: bad, normal, good. Bad/red adds nothing to your power, yellow (unlabeled) adds 20, green/normal adds 40, blue/good adds 60. Hence, at level 1, you can score anywhere between 30 and 90 based on how you do on the bar.
Enemy attack power appears to be different. It appears to follow this formula: (level x 10) + 90. Hence, pink-haired girl at level 2 is capable of 110 damage. It appears that random numbers are picked between 0 and their top power, but I have never seen an attack power lower than 7 from one of the enemies. This is curious!
Also, "escape power" of enemies seems to be calculated based on a higher range; they frequently pull numbers larger than the above formula. I can't quite make any guesses at how the maximum is determined for that, but your attack power in regards to catching them is identical to your attack power in regards to 'knock'ing them.
PROTIP: if you can always hit goods, you can fight people 2 levels below you without ever getting knocked down. If you are 7 levels above the enemy, you will always knock them down, even if you hit bads every time.
For the main fighting/seduction combo mode:
Your "attack" is calculated as follows: (Level x 5) + bar modifier + action modifier
Bar modifier works the same as above! Different actions have different effects on your attack, as listed below. I am pretty sure bras and panties have the same effect, as do all items. Also, every action appears to be equal regardless of what body part you try it on (except for coat vs. underwear).
Coat: +30
Bra/Panties: -20
Items: -30
Knock: 0
Touch: +20
Kiss: +20
Whip: 0
Running: +20
So running, touching, kissing, and removing coat all add to your power, while using items or removing underwear lower your power, and attack-type moves do not modify it at all.
Enemy defense is all over the place and I think the highest possible value rises by 10 each time as well, but it is different from in normal fighting. I can't speak to this really.
Stats:
Exp to next level = (level x 6) + 12
HP = the exact same as experience to next level
This is pretty much the end of the scope of my basic algebra skills/desires to reverse engineer the formulas. Experience gain/enemy arousal/enemy approval after coitus are beyond the scope of this guide.