Who claims he wouldnt have a character?
While he doesn’t have a focus often in One Piece, he got characterizations from time to time.
First like you already shown, he is extremely close to his vice, Killer. They are more like brothers, stick together since their child hood. He and his crew care a lot about each other, not like most other pirate crews.
Another example for that is, when Killer has his laughter, the whole crew decided to laugh with him, so it is not embarrassing for him. Means, they care about his feeling and they want him to have to confidence he is used too. They are, like the strawhats, a bunch of friends, not just a random crew.
Another important part, Kid has more willpower than most of the characters in the story. Maybe even more than Luffy. While strawhats, and most importantly Luffy, got gifted everything around them, they got legendary trainers, legendary weapons, legendary wood for their ship, legendary luck (f.e. with Kuma, Dragon, Garp…), they get assistance from literally every person around them, Kid never got anything of that. But he still made it to the exact same point and took down an Yonko (yes, together with Law). So against all the odds, which all went pro Luffy; he made it this far. Even tho he lost an arm against another yonko, he got imprisoned by Kaido, he got backstabbed by his alliance, his best friend /brother in mind got tortured and so on, he still has his head up, fight for his goals, never lost his will and made it to the same point as Luffy, which had all luck, odds and plotarmor on his side. Which is incredible impressive!