The term "Haki" was first coined and used much later when Blackbeard was commenting on Luffy's bounty in Jaya, and was hinted at during Shanks' visit with Whitebeard. The underlying concepts of Haki were introduced during the Sabaody Archipelago and Amazon Lily Arcs.
Haki wasn't properly introduced until the Marineford Arc, just before a two-year time skip that theoretically marks the halfway point of One Piece, but it was hinted at several times before this arc. Haki is a way of utilizing a person's spiritual energy in offensive or defensive manners.
The fans got the first visual representation of Luffy's Conqueror Haki in episode 389 of the anime. In this episode, Luffy used his Haki for the first time against Motobaro in a fight against Iron Mask Duval.
Zoro is a very powerful fighter and he possesses all three types of Haki. Zoro was able to use Conqueror's Haki unknowingly against Kaido. Biggest Feat: With his newfound power, Zoro managed to defeat the right-hand man of Kaido, however, his biggest feat is scarring Kaido.
Nami is also a competent fighter, although among the weakest in the crew. She mainly makes use of her wits in combat to get the better of her enemies. Surprisingly, One Piece 1058 showcased her using Conqueror's Haki in what appeared to be a gag scene.
Busoshoku is perhaps Luffy's most used Haki as he commonly mixes it with his Devil Fruit abilities to increase his offensive and defensive potential.
4/12 Armament Haki Subtype 4: Advanced Ryuuou
So far, only two characters have shown the ability to use it, Rayleigh being one, and Monkey D. Luffy being the other. Although Rayleigh seemed like an expert at using it, Luffy has only managed to use it subconsciously.
Rayleigh is one of the very few people in the world who can use all three types of Haki. He is also a capable teacher, as he taught Luffy about each type of Haki and trained him in the basics.
Conqueror's Haki, also known as the Haki of the Supreme King, is the rarest form of Haki that only a few people in the world of One Piece are born with. Unlike the other two Haki types, Conqueror's Haki can't be learned through training and is only possessed by those who are born with the spirit of a king.
Kenbunshoku Haki, one of the three Haki types in One Piece, grants the most powerful pirates unfathomable abilities. From sensing people's morality and emotions to being able to predict movements in battle, this form of Haki allows users to tap into a sixth sense that provides impressive precognitive abilities.
Whitebeard was the strongest person in the One Piece world when alive and that's enough to think that he had all the three Haki types. As confirmed by Eiichiro Oda, Whitebeard was a user of all the three Haki types and this has also been seen in the manga quite clearly.
Roronoa Zoro was trained by Dracule Mihawk, the strongest swordsman in One Piece, during the two-year timeskip. The training focused quite a lot on Armament Haki, which is why Zoro's usage of this power is so refined at the moment.
Monkey D. Luffy has mastered all three types of haki, including Kenbunshoku Haki (Observation Haki), Busoshoku Haki (Armament Haki), and Haoshoku Haki (Conqueror's Haki). He can even use several subforms of the hakis, which makes him one of the most powerful haki users in the One Piece world.
Armament Haki is colored black so that the audience can visually identify when someone is using it, as opposed to having to guess or be outright told in every fight.
Sword attacks with haki can hurt Buggy but can't bleed him as haki can't nullify DF power in which he can't be bleed.
Dragon is almost certainly going to be a user of Conqueror's Haki as well just like his son. In Marineford, Luffy's Conqueror's Haki was attributed to Monkey D. Dragon, meaning he is already implied to be its user despite not having used it so far. In the future, fans will certainly see him make use of this ability.
While fighting Monet, a Logia user, on Punk Hazard, Zoro was able to slash her on the cheek, displaying the use of Busoshoku Haki, after saving Tashigi. Zoro particularly specializes in Busoshoku Haki.
The Haki Haki no Mi (覇気覇気) is a Paramecia-type Devil Fruit in which the user is able to see all three types of Haki, as symbolized by a different color. Haki (覇気) meaning “Ambition”, in both Viz Manga, and the FUNimation dub called the Color-Color Fruit and it was eaten by Gozu.
Sanji is an extremely skilled Haki user. During the two years he spent at Maiden Island, he trained his usage of both Armament and Observation Haki. Unlike Zoro and Luffy, Sanji wasn't taught this skill by anyone. He managed to learn all by himself and that is certainly an incredible feat.
She hasn't shown any usage of Haki (yet)
For instance, Armament Haki will coat weapons and body parts with a dark shade of red or purple. It also gives them a very shiny appearance. None of Robin's attacks have been seen using Haki. Anytime she replicates limbs with her Hana Hana no Mi, she never applies the coating.
Heracles is a strange warrior that used to reside in the Greenstone forest in the Boin Archipelago. He trained Usopp in the use of Pop Greens for two years.