Killer is more difficult and works a lot through headcanons since we've spent so little time with him. I'll stick to what's canon for this little passage.
It's true that Killer is smart, knowledgeable and spends a lot of his time trying to control Kid's wild attitude. However, I feel like most people forget that Killer too has a wild side.
When we were introduced to him, he was clashing against Urouge apparently because yes. We saw him ready to kill a woman and a child with no hesitation.
He's not a gentle soul that would just help anybody on his way. He's willing to die for Kid and he's probably protective of his crew but that's about it. He'll probably be protective over his s/o too but, honestly, I see him putting Kid first, even before his loved one.
Also, he's self-conscious of his laugh (so he wouldn't willingly laugh) and possibly of his face.
Oh, and he trains a lot to get stronger.
