Sir Crocodile is one of the longest running and most noteworthy primary adversaries of the series, as he was the first enemy to hand Luffy a complete and utter defeat, as well as one of only two who has defeated Luffy more than once.
He was originally introduced as a Shichibukai but was later stripped of his title after attempting to take control of the desert kingdom Alabasta.
Thanks to Luffy, he managed to escape from Impel Down and went to Marineford to participate in the war. After the war, he and his subordinate Daz Bones decided to head out to the New World.
Ivankov found out a precious weakness of Crocodile’s, which he later used as blackmail to ensure that the known villain would not betray him.
There are of course a lot of theories about what the secret might be, the most popular is that Crocodile used to be a female.
Myth: Ivankov knows a secret from Crocodile’s past, that nobody else knows, and Ivankov can change people’s gender, leading to speculation that Crocodile used to be a woman and had Ivankov change her to a him. In Episode 0 we only see the back of Crocodile’s head, suggesting that he was different back then and that Oda wants to keep it a secret for now.
Fact: So far, nothing has been confirmed. In the SBS for Volume 63 Oda drew the Shichibukai as children, and Crocodile appears to be depicted as a boy, although some have speculated that Crocodile could be a girl who dresses like a tomboy, but Oda has not yet stated anything one way or the other.