J
For me, he's among the best WG members introduced so far with a cool design and a unique fighting style. He's likely to have a decent background story though I don't expect it to be as good as Laws. I don't understand why people even think he's weak. Losing to Zoro who's probably above any other SN sans Luffy and Kid doesn't make him weak ffs. He lost to the second most important character in the series. He's not important enough overshadow the main characters in terms of the only thing they're good at. Killer totally lived up to his WG hype as far as I am concerned.
Oda had to nerf Killer 3x for Zoro to win