If he had to describe it, the life he'd led before he met the man could be called "Twilight"
And then Tendou Souji appeared. Like the sun rising over the horizon, eventually outshining everything. Above everything.
At first he'd been dazzled by this perfect light, but he'd groped in the reverse darkness for the thing that was most important. And eventually, his eyes had adjusted to the light that was now a prominent factor in his life. He could things more clearly now then he'd ever had before.
He was grateful, actually. He'd always been drawn to strong people, perhaps hoping that some strength would rub off on him. That was part of the reason he'd joined ZECT in the first place. He knew he could learn. He wanted to learn. He needed to fight, to prevent some other person from losing their little brothers.
So, he attempted to make friends with the man who walked Heaven's path. No matter what he could say about his personality, Kagami had to admit he was amazing person. He was envious of his perfection at first, but eventually realized you can't be jealous of a sun for shining. Annoyed, yes. Even later, when they'd shifted into some sort of camaraderie, that light still burned. But that was the line between them. There were things that Tendou was resigned to do that Kagami wouldn't, couldn't do. He'd told Tendou as much. The only person he could ever be was himself. But, at least he knew himself well enough. There had to be somebody to rush in blindly, sometimes.