If you're reading this I know it's because your a fan of the OHSHC. Maybe you've read the Manga, if that's the case you'll know all this. If you haven't read the Manga then this will be very informative to you.
In the Manga the story goes so much farther. I'll give the best details :)
Kaoru realizes that he, Hikaru, and Tamaki are all in love with Haruhi. Haruhi was hanging out with Kasanoda (the dude with the red hair that knows Haruhi is a girl) and Hikaru gets really jealous. He tells Kaoru that he doesn't understand why it makes him so mad to see Haruhi with another guy. Kaoru smartly tells him that he sees Haruhi as a girl, not just a friend, and that he loves her. Hikaru tries to deny it at first but then stops talking and lets the truth sink in. Kaoru then tells Hikaru that he also loves Haruhi and he wouldn't lose her to a moron who couldn't tell that he was in love with her. (He doesn't say those words exactly but pretty much he calls him stupid lol.)
Hkaru and Kaoru are so mad at each other that they each refuse to go home. Kaoru sleeps over at Honey's while Hikaru sleeps at Mori's.
Kaoru, being the brave soul, asks Haruhi on a date. He takes her to a fair the next day. She can tell he's sad but he says to not worry about it and asks if he can hold her hand. Haruhi agrees. They go on a nice little date, holding hands the whole time. Nearing the end of the date they are sitting at a table. Hikaru, having found out about their date (from Honey and Mori, which Kaoru planned) decided to stop the date. Kaoru sees him behind Haruhi and leans in and kisses her on the corner of the mouth. She's a little too shocked to do anything so she just looks at him. Kaoru tells her that he's in love with her. He also tells her that no matter how much he cares about her he loves Hikaru more. Haruhi, pretty confused, just says ok. Kaoru asks if they can still be friends and she agrees.
Hikaru, seeing Kaoru kiss Haruhi runs away, furious and hurt. He goes back to Mori's house and hides in the room he's been staying in.
Once Kaoru takes Haruhi home he goes over to get Hikaru. Hikaru is so mad at Kaoru he won't even look at him, so Kaoru kisses him full on the lips. He loudly announces that he shared Haruhi's kiss with him and so he can't be mad.
Kaoru takes Hikaru home and tells him that although he stepped aside that Tamaki might not. He tells Hikaru to fight for Haruhi and promises to be on his side of the battle through the whole thing.
I'll write more about the story later :) Hope you enjoyed this!