He knows everything about her, right from her favorite books to her favorite bra.
She knows everything about him, right from his favorite soccer club to his favorite x rated websites.
He will complete her English homework, even at three in the night.
She will arrange an Armani suit for him, even if it calls for flirting with ugly guys'.
He has her picture in his wallet.
She has his number on speedial.
They talk to each other all the time.
They talk about each other when they don't talk to each other. 
They discuss everything from periods to playstation.
They have tasted alcohol & then thrown up ...together
They have bunked countless tuitions...together
They can't live without each other
They don't love each other
They are JUST FRIENDS...