Boys:
Get really mad at your girlfriend every time she talks to the guy who likes her. Even if he's not talking romantically, pretend you don't understand that. When the two of you fight and she goes to cool down, turn around and cheat on her with the other girl who likes you.
Never admit that you like your girlfriend or that she's even your girlfriend.
When anybody walks in on your romantic endeavor, immediately shove her out of your arms.
Sneak into her room and break her alarm clock.
Speak loudly and carry a really, really big sword.
Argue with your perverted monk friend and get teased by a little fox dude.
Call your ex-girlfriend's (who you cheat on your girlfriend with) elderly younger sister a hag
Frequently stumble on her naked/half naked and act very flustered while lending her your shirt to make into a little dress thing.
Protect her from your older brother, your arch enemy, and various other demons.
Wear red balloon pants
Constantly remind your girlfriend that you only keep her around as a Shikon Shard detector.
Have a dead mother.
Get quite upset or defensive if she starts crying and yell at her to stop crying.
Accept huggles if you're upset, and if she doesn't offer one when you're upset, huggle her anyway
Bother her if she goes back home for more than three days at a time
Girls:
Constantly forgive him for everything because you love him even when you know you shouldn't
Place a curse on him and use it to stop him every time he gets too violent
Complain about him to everyone back home
Allow your friends to pressure you into two-timing him with a guy back home
Get mad or upset when he two-times you
Get into lots of fights with him
Tell people how much like a cuddly puppy-dog he is
Glomp him whenever you want to, or if you're afraid
Protect some of your friends from him
Fiddle with his ears when he's unconscious
Get hit on by random monks and other guys
Occasionally help your boyfriend with his fights by firing magic arrows at the enemy
Run to him for protection every time you are attacked by a demon, his brother, or his arch-enemy
Try to steal him from that other girl he's in love with
Get undressed in his vicinity a lot so he can accidentally stumble upon you
Wonder if he's actually a boy or not (after all, he is half demon)
Have an annoying grandfather who likes to tell rambling old legends
Have big boobs
Both Boys and Girls:
Fight, like a lot. I emphasize this. Fight. Fight. FIGHT!!
Be very good looking and have big anime eyes, and long pretty hair.
Go on journeys in Feudal Japan hunting for the Shikon Jewel.
If you have no convenient dry well/time portal, go on journeys in your backyard and pretend its Feudal Japan and you're hunting for the Shikon Jewel
Never give up on each other, even if you already chose to be in love with a different person
Travel around with a small posse of demon fighters
Never, EVER, at all under any circumstances can you kiss, at least not until the end of the series