I do not own Ouran.
From his earliest memories, Kaoru had been a fixture in Hikaru's life.
There was a brief period when he was a small child that Hikaru had questioned the existence of another him; someone who followed him around while he explored the large mansion in which they lived. It never occurred to him that they were not two parts of the same person, as their mother frequently called both of them either Hikaru or Kaoru at random.
Kaoru had always been quicker to pick up on things like the fact that they were twins. It must have been why every time Hikaru turned back to look at him, he would just smile warmly, as though he had always known that they were special. It was a reassuring smile, one that calmed all his questions and doubts about the matter.
Hikaru loved that smile. Even before he finally understood what words like 'identical' and 'twins' meant, he had taken to falling into step with Kaoru instead of walking ahead of him like before, just so he could see it. When Hikaru did this, Kaoru's smile was brighter than ever, and he couldn't resist returning it.