It wasn't hard, to notice, once you knew to look for it.
If someone were to tell you that Ichigo Kurosaki and Grimmjow Jaegerjacques were extremely similar, you might laugh it off, at first. How absurd, you might say. They're nothing alike. One is a hero, and the other is a Hollow. And their personalities! So different.
But then you'd start thinking. Hmm, maybe they were right. And you'd begin to look closer. And you'd see it.
You'd see Ichigo's temper, which he hasn't shown as much recently - it's still there, though, that loud and angry boy with the permanent scowl. The fierce fighting spirit, and that love of fighting. You'd see the shouting, cocky boy that is the strongest boy in Karakura - the boy that called for ambulances for his opponents before the fight even started.
You'd see his confidence, and his sarcasm, and his over-the-top reactions - and his determination and sheer stubbornness. His refusal to be given orders and his lack of care for proper authority and being polite.
You'd see a lot of what Grimmjow is in Ichigo, really. It's not hard to see the brash Espada in the substitute shinigami, once you look.
It is a lot harder to see the shinigami in the Hollow, but he is there.
He's there in the rare moments of solemnity, in the honor code that most of the others don't seem to have. The desire for fair fights. The desire to fight and be strong.
The two share determination and stubbornness, hotheadedness and reckless courage. They share a disregard for politeness, and a refusal to listen to orders. They share a love of fighting, and skill at it, and a desire to get stronger.
There are many similarities between the two, though they are not alike exactly.
If there would be a way to describe it, it would be to compare it to a distorter mirror, reflecting one as the other in its rippled glass - not exact, but close.