Kaoru is the mature twin. He's the one who analyzes people and predicts situations before making a move. He's the calmer and patient of the duo.
Hikaru on the other hand is the rash one.
But sometimes when he's alone; when he's in the sanctity of their world without the company of his brother, and he absolutely hates these moments because he hates being apart from his brother, hates being consumed in silence so noisy it threatens to shatter his ear drums (but doesn't really, because it's not) and by shadows encroaching in from all sides, hates being alone (even in the bubble they'd created for themselves)and also, because in these seemingly insignificant spans of time he too, like Kaoru, starts to observe people.
His eyes wander over to the Host Club sparing each of them a glance but lingering for just a little longer onto the intruder; (because in his opinion she is just that no matter how much his heart tries to deny it) Haruhi and onto the smile adorning her lips and onto the tiny sparkle in her large doe like eyes which rest on their oblivious blonde, violet eyed sempai.
And then he detests these moments even more because in these tiny spans of time something in his chest; his heart, clenches and his mind registers this really uncomfortable feeling gnawing at his insides, mangling them into a mushy heap.
And then, because he misses his brother so much and because there's this stupid speck of dust that decided to dive into his amber irises and also because (mostly because) the pain gets so unbearable, water clinging to his drenched lashes trickles down his flushed cheeks.
To him the first two reasons seem more valid, because people who cry 'cause their 'hearts get broken' are totally lame.
(And Hikaru Hitachiin is anything but lame.)
And then he decides to give up on her. Not because he's tired of waiting for her, because he would never get tired of waiting for her. Not because his bruised heart cannot take any more of the searing pain. Absolutely not! Because he is strong. His heart is strong and something so utterly stupid like an unrequited love can't weaken it. But because, the look in Haruhi's eyes when she gazes at the Idiot Lord is the same as the one present in his eyes when they rest on her. And because she smiles the same way he does when he's thinking about her.
Because she loves Tamaki the same way he loves her, he lets her go.
Because if her love for Tamaki is as strong as his love for her then absolutely no one, not even Hikaru, can rival it.