The main character (Cid) isn't a Rance-like dude, basically he's pretty Beta nice guy and he wants to marry the daughter of a fighting gym dojo, but ol girlfriend's dad isn't going to give up his daughter to a pussy so he makes a condition: win the generic RPG tournament and he'll prove that he's strong enough.
So dad isn't going to send his daughter in as Cid's collateral, after all, he doesn't want his daughter to get banged by the 1st fat ugly jerk Cid loses to. Instead his aunt goes with him as collateral and his girlfriend goes with another dude that's the current strongest guy at the gym.
Whenever you win a fight, you get the choice of whether to just let the collateral girls go and... uh, be a Rance-like gentlemen. Guess which choice will bite you in the ass later!
Of course this is an RPG so you know the main character is going to win and what a concidence, it turns out the guy who's entering with his girlfriend is the last guy he has to beat! After that they have sex and they're all like "ok, time to go home and get married, you go first but I got to collect the prize money"
And here is the big plot twist of the game: As it turns out, the owner of the tournament has a ulterior motive for holding the tournament. Every "winner" gets poisoned with an addictive drug by her and requires constant supply and raping of innocent victims or they'll go berserk and die. The girl that enters with the winner is held hostage and used for that purpose, as logic dictates that only someone who really trusts the guy would be willing to risk themselves to be entered as collateral that way and thus make good hostages. The owner sends the "winners" off to fight down in a deep dungeon to free her sealed demon boyfriend (or something like that), if they die, tough luck, new suckers every year. Once again there are a bunch of moral choices on whether you should just give in to your urges or try your best to fight it as you're forced into this shitty situation.
As another plot twist, eventually you play as Cid's girlfriend, who isn't stupid and realized that he didn't decide to disappear off the planet for fun. She manages to defeat the villianess's plan but Cid is still poisoned. This is where the moral choices that you made earlier in the game kicks in. If you're pure of heart throughout the game he manages to purge the poison from himself with the help of his girlfriend. Otherwise get ready for a bad ending.
So they go home and get married and the dojo people are like "AH HA, actually it was my plan to turn you from a beta wuss into a super strong dude after all! Except for the whole kidnap and hostage part, that was unintentional."
