Kid is first and foremost, a foil to Luffy. He has Luffy's will power and drive, a love for his crew similar to Luffy's, and acts before thinking.
However, he lacks Luffy's ability to accept and help others, fostering a survival of the fittest mentality, but that's what holds him back. For example, Kid only focused on escaping from Udon while Luffy took it as an opportunity to grow, helping and accepting aid from another prisoner to grow stronger.
And now as both head up to confront Kaido, Luffy comes armed with the haki training he had with Hyogoro, one that focused more on technique rather than power. Meanwhile, Kid is heading up to Kaido thinking bigger and stronger metal is the answer.