Like all of the other arrancar, Grimmjow was originally a hollow. Trying to satisfying a perpetual, voracious appetite for other souls, Grimmjow spent most, if not all of his time fighting and consuming other hollow. In doing so, he quickly became a and then ascended to an , the second strongest class of hollows. As an adjuchas, Grimmjow's form is that of a panther (which is why in resurrecion Grimmjow resembles a panther). Continuing on his path of fighting and consuming other hollows, he eventually ran into , , , , and , who were adjuchas as well (with the exception of Nakim) at the time before becoming arrancars. In the encounter, Di Roy initially provoked Grimmjow, who swiftly retaliated by striking him and then a few of the others. Shawlong and the rest of the group recognized Grimmjow's strength and invited him to travel with them as their "king." Grimmjow agreed to their proposition and traveled with them, consuming 3000 hollows in their time spent. Shawlong and his companions realized that their evolution must have come to an end, having not gained any power after feasting on their 3000th hollow. They collectively elected that Grimmjow should eat them so that they could advance to as a part of him. Though Grimmjow agreed, future events suggest otherwise, seeing how they all became and Grimmjow's Fraccion.
Calling both Ichigo and Grimmjow honorable might sound as a stretch, but they can not be considered as the pure good guy and the pure evil guy either. We already mentioned the existence of Hollow Ichigo, right? Anyway, Ichigo gets disgusted when Yammy calls Grimmjow trash. He also spares the life of his rival on episode 167, where Ichigo is finally able to surpass Grimmjow but refuses to give the fatal blow.