You can date any of the characters in a game usually female but there are Dating Simulations for girls with male characters, there's some sort of relationship points system, in which every action you make and everything you say will make him/her happy, sad, angry, etc. and this can increase, decrease, or leave the points unchanged. The more relationship points you have, the happier he/she gets, and the better the ending with that character which could contain anything (cute pics, hot pics, secret things of a naughty nature, even minigames). This is for a normal dating sim.
Sister Princess and Sister Princess 2 are for the PSX
Sister Princess: Repure is for the GBA
Aria has 3 hot sisters (Chikage, Haruka, Sakuya)