This is another thing that doesn't require much thought.

1. Innocent Sweetheart and Dominant Jerk.

Do this one by making the MC an innocent sweetheart who wouldn't even dream of swatting a fly. Make the MC's crush a super jerk who wants to be in total control of everything the MC does.

2. Dumbass and Smartass.

Do this one by making the MC a complete idiot who is always getting him/herself into horrible situations. Make the crush really smart and snarky, always rescuing the MC and giving him/her a hard time about it.

3. Model Student and Delinquent.

Do this one by making the MC a perfect student who never breaks the rules and always does all of his/her homework. Make the crush a bad student who is failing every class and has a rotten attitude.

This one can also be done in other settings too, like a workplace. MC can be a model employee who always meets deadlines. The crush can be a bad employee who always pawns his/her work off to other people.

Just remember the well-know line that "opposites attract." Make the MC and their crush as different as possible. Whatever personality the MC has, give the crush the polar opposite.

Now this isn't the end of personalities. No way. You also need personalities for their best friends.

The MC's best friend has to be super annoying and constantly trying to get the MC to hook up with the crush. S/he can also be super slutty.

The crush's best friend should either be a total whore with a bad attitude or actually a nice person who sympathizes with the MC.

With personalities picked for these characters, no one else in the story really matters.