It was a bet.

To get Kim Saehee, the hardest girl to get. Will L fufill his needs? 

"Back Off, She's Mine!"

--

(You) Kim Saehee

Age: 17

She doesn't take an intrest for boys. None of them intrests
her. She is the queenka of the school as boys swoon over 
her and girls dream to be her, although they might feel angry
that she is the queenka, they don't have the guts to go up
against her, as she always hangs out with Kim Jaejoong, who
knows kungfu. 
 

Kim Jaejoong

Age: 18

He has a cold personality and doesn't talk much. He might be
cold on the outside, but he had been your best friend from 17
years ago. Girls swoon over him but he doesn't pay attention
to any one of the females. Wonder why I didn't say other than
his mother? Because he's an orphan. Both Saehee and Jaejoong
don't have mothers and they are orphans. The nurses loved them
so much that they saved enough money up to buy a whole house
for them to live in.

Kim Myungsoo / L

Age: 18

He has a cold personality (too), he hangs out with his only friends,
Infinite, usually talks a lot with them. Unlike Jaejoong and Saehee,
he has parents. He is wealthy and usually insults others, he is one
of the bad boys in school but he doesn't pay attention to females or
more specifically, you, until the bet came along.