Boa Hancock is one of the most powerful characters in the world of One Piece. Introduced before the two-year timeskip, Hancock is the captain of the Kuja Pirates and the Empress of Amazon Lily, a secluded island in the Calm Belt of the One Piece world. The Kuja are known to be a tribe of powerful women all of which can use Haki. This tribe admires strength as beauty and Hancock, being the most beautiful of them all is, naturally, the strongest on the island.
At a very young age of 18, Hancock was made the captain of the Kuja Pirates. At the same time, she received an 80 million berry bounty after her very first voyage and, immediately after, she was granted the status of Shichibukai. As confirmed in the latest Reverie, Hancock is no longer one of the Warlords of the Sea and, thus, is hunted once again. The Marines reinstated a bounty on her head and One Piece 1059 revealed this reward to be 1.659 billion berries for many reasons.
Hancock's Powerful Haki
Hancock belongs to the Kuja tribe which is known throughout the One Piece world for their skills and mastery of Haki. This was made clear to the fans when Luffy was sent to the island by Bartholomew Kuma, one of the former Warlords of the Sea. Even people who were not a part of the Kuja Pirates were capable of using Haki and storing it within their weapons to increase their attacking potency. Those who were a part of the crew were revealed to be even stronger. Hancock's two sisters were incredibly powerful and gave even Luffy quite a difficult time with their Haki. Evidently, Hancock being the strongest of all, has a powerful Haki as well. She is a user of all three types of Haki, making her one of the very few people with these skills.
When it comes to Armament Haki, Hancock is certainly more than impressive as Smoker gave special attention to her Haki and named it to be the "Haki of the Kuja" immediately after being faced by it. Her Observation Haki skills were revealed to the fans in the Paramount war, especially towards the end where she was able to sense Luffy and have Salome track him down – something that even the Admirals of the Navy could not sense. Hancock, being a user of all three types of Haki, has the power of Conqueror's Haki accessible to her as well.
Hancock's Mero Mero no Mi
Hancock also the user of a very powerful Devil Fruit called the Mero Mero no Mi. This Devil Fruit belongs to the Paramecia class and is known to be one of the most powerful fruits in existence. Its abilities work based on Hancock's beauty. Anyone attracted to her, regardless of their gender, can easily be turned into stone by her using techniques such as Mero Mero Mellow. Furthermore, the petrification can only be undone by Hancock and even new users of the fruit cannot help those who have been petrified. At the same time, Hancock has several other techniques that don't require attraction to work. Slave Arrow, Perfume Femur, Pistol Kiss, among others, turn even inanimate objects into stone on contact.
Evidently, Hancock's Devil Fruit is one of the most powerful ones that Oda has introduced. This is one of the reasons why Blackbeard, one of the Four Emperors of the Sea, had his eyes set on this Devil Fruit as well. As the Shichibukai system was abolished, Blackbeard made his move to get his hands on her powers. Thanks to Rayleigh's interruption, the situation was de-escalated.
Why Hancock's Bounty Is 1.659 Billion
Hancock is an incredibly powerful pirate whose skills go beyond the level of the Yonko Commanders. Her Haki alone makes her a very dangerous fighter. Being a user of Conqueror's Haki and a specialist in this power means that Hancock can possibly coat herself in the Color of the Supreme King as well. As mentioned by Kaido, only a handful of the strongest pirates can do this. Furthermore, Hancock's Devil Fruit is rather special and this nature of her fruit makes her even more dangerous.
All things considered, Hancock is an exceedingly rare class of pirate with powerful Haki and a dangerous Devil Fruit. As such, the world government has placed a 1.659 Billion berries bounty on her head, which is extremely high. At the same time, it must be remembered that Hancock is not a part of any organization such as the Cross Guild which, if she were, would certainly have inflated her figures, as seen with the likes of Crocodile, Buggy, and Mihawk. She deserves her bounty and currently, she possesses one of the highest figures in the One Piece world.