Sol exists in a kind of suspended animation -- a stagnation. His body does not age, it always heals itself right back up to its most optimal functioning point, he does not age -- he walks forward, but maybe it's just everything else around him moving past him and he's not really progressing. It's a kind of complacency that comes with being practically immortal and indestructible (that comes with being no longer human).
That is, until he meets (and falls in love with) Ky Kiske.
Ky doesn't know when to quit, when to stop, when to cease trudging tirelessly endlessly forwards, even when his heavy boots are caked in mud, grime and blood; his coat weighs him down with more mud, grime, rain, sweat, and more blood; when his hair clings to his eyes with sweat, rain, and grime, and he barely has any energy left to do anything else but keep himself awake -- still, he always trudges forwards. And somehow, he manages to convince everyone else around him to trudge forwards as well (towards a better tomorrow).
Sol had heard it whispered once - that there was no one else like Ky Kiske.
Even with a hundred and seventy years of existence under his belt (and the hundreds upon hundreds of men, individuals, creatures, he's encountered) -
He thinks they're right.