Number 6 has great respect for Knuckleduster when he was the hero O'Clock, to the point that he has a huge poster of him in his apartment. When he was still an experiment he watched when O'Clock's career fell apart and lost his Quirk. His admiration is such that his characteristic scar on his face is self-inflicted, as a tribute to the one that Knuckleduster has. He is so proud of this scar that he can not bear to have another injury on his face. His goal is to become the true successor to O'Clock, considering multiple possible names for himself, including: "O'Clock II" or "Two O'Clock".
Knuckleduster had no relations with him before investigating the Villain Factory. He doesn't know who he is or what his origin is, except that he is an agent of the Villain Factory and that he is in possession of his stolen Quirk Overclock. When they finally meet in person, Knuckleduster considers him nothing more than a villain, one that he plans to eliminate, so when Number 6 asks him to recognize him as his successor he flatly refuses. Despite being enemies, and having no qualms about killing him, Number 6 considers Knuckleduster the best one, much better than the Pro Heroes, even All Might.