Kaoru always knew his brother was the one for Haruhi. Hikaru notices Haruhi before everyone else, does stupid things around Haruhi, but most of all, he matures around Haruhi. He opens his eyes a little wider every time he's with Haruhi. It was... comforting to know that his brother was in good hands.
Kaoru knew that their magic spell began to fade when Tono realized Hikaru's affections. But, they stayed together even without a spell. All of them supported the relationship, even Tono who wouldn't talk to anyone for a day. And as Kaoru watched his brother dancing with Haruhi, their eyes locked on each other in a way that spoke volumes, he smiled and turned away. Hikaru was happy, and though the spell was broken, they stayed together. That's all Kaoru could ever ask for.
Hikaru/Haruhi from Kaoru's POV. It was written for the request thread at my forum. (Link on profile.)