Tho in the crew is needed a balance where there should be soft hand, Luffy, and a strong hand, Zoro, who would take care over the situation from the colder side as it was the best shown while the Water 7 saga. First mate doesn't have to be charismatic or stuff like that. For example Rayleigh while he was still a pirate and a first mate of Roger was a colder and more stoic persona in the crew who would take over the more serious topics or situation.
fun fact: at the beginning Oda planned that Usopp would be a first mate