Warren D. Luke also known as and commonly referred to as Diamond Back is a Pirate captain born and raised in the Kano Kingdom located in the West Blue Luke is the son of current Navy Admiral Warren D. Ralph and nephew of the Revolutionary Brown D. Newton. Luke is the son of a female member of the Chinjao Family, a gang that resides in the Kano Kingdom and are the leaders of the Happo Navy. With this background it is natural that Luke possesses the fighting style called Hasshoken.
Luke learned Busoshoku Haki from a young age because of his father. When Ralph took vacations from his duties as a marine, he would go and spend time with Luke in the Kano Kingdom. During these visits, Ralph would teach Luke several skills that he could use later in life and Busoshoku Haki was one of those skills. Ralph would teach Luke bits at a time and when Ralph left Kano, he left Luke the task of training in the skills he taught him so improvement could be made. Every time Ralph would come back, Luke displayed outstanding skill in Busoshoku Haki for his age at the time. Ralph would train and teach Luke new things then again leave; Ralph would do this every time he went on vacation until he felt Luke was skilled enough to handle himself. When Ralph was gone which would be anywhere from 6 months to several years at a time, Luke trained his Haki every single day. Around the time Luke reached his early teenage stage is when he obtained the black coating of Busoshoku Haki. Upon first seeing Ralph use this coating of Haki this was Luke's goal and when he was able to do this he wanted to move to the next level just like his father. He trained to do this greatly increasing the power of his Haki at its current state while at the same time working on his Hasshoken.
Luke also was assisted in his own training by various members of the Chinjao family in Busoshoku Haki. Luke's motivation was to be able to mimic what his father was able to do, this drove him to intense training with Haki in order to achieve this level or armament. Eventually, Luke achieved the black coating of Busoshoku Haki. Since he achieved this at an early age, as he got older and continued use of Busoshoku Haki it naturally became stronger with each usage. Luke had a grasp of the potential of this form of Haki and unlocked other ways to use it. Like his father and several other Busoshoku Haki users, Luke can extend his range of attack by using Busoshoku Haki in the form of a manipulative energy like substance that he can use as an extension of himself. Shockwaves of haki that extend his range which allows for him to battle against opponents who's bodies are augment by devil fruits or fight from a distance.
Luke's specialty however was using Busoshoku Haki in the form of shields that can protect himself and others from attacks or reduces the power of attacks aimed at him. These shields can take massive damage from attacks such as Hasshoken and attacks stronger; how much damage Luke's shield can absorb all depend on how much energy he inputs into the shield. Hasshoken is the reason Luke decided to experiment with using Busoshoku Haki in this manner. Hasshoken is known to be able to penetrate through defenses such as shields, however a shield made of Haki is way more difficult for the vibrations to slip though. This was shown during the war on Marineford 50 years ago when an attack from Edward Newgate using his Gura Gura no Mi abilities which could manipulate vibrations were blocked by the combination of Akainu, Kizaru, and Akoiji's combination of Haki. Luke heard of this event and was able to build his own shield of Haki to ward off attacks of nearly that caliber.
Through continued usage of Busoshoku Haki, Luke was able to specifically coat the insides of his body in Haki such as his bones, organs, and muscles. Luke is very proficient in coating specifically his muscles in Busoshoku Haki. Not only does Luke substantially increase overall power, but his defense in majorly boosted. Muscles are flexible and are able to withstand stress of massive proportion if trained enough to handle said stress. With Luke's rigorous training, his muscles are able to withstand a large amount stress already. Specifically coating his muscles in Haki greatly increases how much they can take and dish out. Luke's usually keeps his muscles coating in a small amount of Haki at all times because of his lack of Kenbunshoku Haki use. However, once he pours a significant coating of Haki on his muscles, physical attacks simply do not affect him. Bullets bounce off, swords break on his skin, and even cannon blasts are futile against his haki coated muscles hence his commonly referred name Diamond Back