He is also currently a member of the Five-0 Task Force and the partner of Lieutenant Commander Steve McGarrett.
Danny grew up on the mainland, in New Jersey. His family consists of his younger brother, Matt Williams, his sisters and parents.
When Danny was a beat cop, his future ex-wife, Rachel Edwards, rear-ended his car to get his attention. As he puts it, "The driving lessons turned into dates, and dates turned into a two bedroom, and she (Rachel) woke up married to a cop."
Later, they had a daughter named Grace. It is not known what happened between them, but Danny and Rachel ended up having a divorce. When his now ex-wife remarried and moved to Hawaii, Danny reluctantly went along to Hawaii, to be able to see to his daughter.
Danny's father is a firefighter in New Jersey. His mother is presumed to be an excellent cook, as her lasagna was, "the last thing I want to eat before I die," according to Danny, until he tried Cocoa Puffs.
Unlike Steve, who is typically calm and in control, Danny is nearly always displaying one emotion or another. The most obvious is anger, as he possesses a very hot-headed nature with a vicious temper to match. He's also the most outspoken of the team and quite possibly the whole H50 Force and HPD, regardless of where he is and who's around to hear him.
Despite his often off-putting, and at times offensive, attitude, Danny is also one of the most openly affectionate characters and fiercely loyal to his friends and family.
Of the four main members of the task force, Danny is the most vocal. As his partner and, as Danny calls him, an animal and Neanderthal, Steve is the person most often on the receiving end of one of Danny's long-winded explanations of what is wrong with the current situation, as Danny expresses a deep dislike for Steve's methods, such as going into a house without back-up, and isn't afraid to tell him so as loudly as he pleases.
Despite his abrasive personality, both Steve and Meka cite Danny as a great cop with good instincts. He is also shown to comfort the victim/victim's family when they are in distress, presumably because Danny is a father.
Although Danny moved to Hawaii, he only did so to stay close to his daughter, Grace, whom he loves dearly. He also spends most of the time trying to impress her or worrying about her safety.
In the episode, E Malama (episode), Danny threatened to kill a business man if he ever endangered Grace again after she and Rachel were victims of a carjacking.
At the end of Loa Aloha (episode) when he confronts his brother about his crimes, Danny chose to say goodbye rather than shoot him.
Danny was shown to be fiercely protective of his family as seen in the episode, E Malama (episode) where he threatened to kill a businessman who had unknowingly endangered the lives of Danny's ex-wife, Rachel and daughter, Grace and in a deleted scene from that same episode, Danny was seen telling Stan that if he ever put her (Grace) in danger again, then Stan would find that the scariest criminal in the world is a cop who didn't care anymore, hinting that if Grace was ever hurt again or presumably even killed, then Danny would go off the rails altogether.
In the Season 2 episode, Mai Ka Wa Kahiko (episode), Danny's dark side emerged with a vengeance where after shooting his ex-wife's husband in the shoulder, Danny shot his ex-partner in the right kneecap much to the horror of Steve, Kono and Chin and very calmly yet coldly threatened to kill Peterson by shooting him in the head if he did not tell Danny where Grace was. This suggests that Danny is willing to do anything to guard Grace, maybe even resort to killing or threatening someone to ensure her safety.
Danny's darker side again emerged in the Season 3 episode, Ho'opio (episode) where he beat up two suspects during the investigation concerning the murder of a young girl and the subsequent disappearance of another, presumably due to the fact that the girls bore a striking resemblance to Grace.
During the interrogation of one suspect, Danny knocked the suspect off his chair, forcing Steve to physically restrain him while during the interrogating of a man and his wife who the team believed were the kidnappers responsible, Danny later hit the man once, prompting Steve to ask for Danny's two badges albeit temporarily. Danny later gave them to Steve and proceeded to beat the man while Steve calmly walked out of the house as the punching was heard.
Much to Steve's confusion, Danny despises Hawaii, calling it a "pineapple-infested hell hole." This point is further proven by the way that he does not care at all what the locals think about him wearing a tie. Danny does not like the water either; it is hinted that Danny does not like the water because he cannot swim.
However, halfway through Season One, Danny is seen trying to surf. In Lana I Ka Moana (episode), Danny tells Steve that he actually used to love the ocean, but started hating it when his best friend Billy died on the ocean while swimming towards Danny to help him.
Danny actually did dive into the ocean once during a case which left Steve greatly impressed and when Steve remarked "Book 'em, Danno", Danny replied, "Book me a towel".
Although Danny denies it, Steve observes that Danny likes Hawaii. When Danny first met Steve, it seemed that Danny hated Steve more than Hawaii itself. Danny also didn't like it when Steve kept calling him "Danno." Likewise, Danny mockingly calls Steve "Steven." Even though Danny tries to hide it, they have become friends, and deeply care about one another, and completely trust each other. Danny also expresses concern about Steve's behavior, as he believes that Steve needs professional help for his inhumane behavior and attitude.
Like the rest of Five-0, Danny is shown to have complete loyalty to Steve.
Since Heihei (episode), Danny's relationship with Rachel is less strained than it was in the beginning of the series, especially in Loa Aloha (episode). Danny is shown to dislike Stan Edwards, Rachel's husband.
In the Pilot (episode), Danny had been living in Hawaii as a cop for six months when he was assigned the murder investigation of John McGarrett.
This brought him into contact and conflict with John's son, Lieutenant Commander Steve McGarrett (2010) who sought to personally investigate the crime himself and as such, this led to bad blood between the two and Steve used his influence to become the leader of the unnamed Task Force.
Despite Danny's misgivings, Steve appointed Danny as the second-in-command of the team and also made Danny his partner in the field.
Unfortunately, things eventually came to a head in the aftermath of a shooting where Danny threatened Steve, Steve responded by placing Danny in an armlock before forcing Danny to apologize.
Danny, on the other hand upon being let go punched Steve and remarked, "You're right. I don't like you" before leaving while Steve was left reeling and obviously stunned that he had underestimated Danny.
However, the team later grew to accept and value Danny's skills as seen during Malama Ka Aina (episode) where Danny gave them information about a crime boss he had presumably encountered or heard of during his days as a cop in New Jersey.
n Ua Hiki Mai Kapalena Pau (episode), Danny fell ill after touching the body of a man to check for a pulse.
This incapacitated him for a time and he suffered convulsions, forcing him to be taken to hospital for treatment. Agent Jenna Kaye later realized that Danny had been infected with sarin. As such, with Danny in hospital, the team turned their attention to finding out who had poisoned the victim and Danny.
Danny eventually made a full recovery and it was later revealed that he was having an affair with Rachel, his ex although they later broke it off for good with Rachel returning to her husband, Stan Edwards.
When Steve headed to Japan to find Joe White and seeks answers in regards to "Shelburne", Danny took temporary control of the team as they investigate a serial killing case and also a possible smallpox outbreak which despite the two team's best efforts ends with the group realizing that a suspect has fled to the mainland. This had Danny and Detective Lieutenant Chin Ho Kelly travelling to Los Angeles along with Special Agent G. Callen and Special Agent Sam Hanna to help stop the spread of smallpox which occurred in the last few seconds of the Hawaii Five-0 Season 2 /NCIS: Los Angeles Season 3 crossover episode, Pa Make Loa and concluded in the second and final part of the Hawaii Five-0/NCIS: Los Angeles episode, "Touch of Death".
In the NCIS: Los Angeles episode, Touch of Death (episode), Danny and Chin arrive in Los Angeles and head to the NCIS: Office of Special Projects to assist Callen, Sam and the OSP team on stopping a smallpox outbreak from happening.
While there, Danny butted heads with NCIS/LAPD Liaison Officer Detective Marty Deeks who jokingly regards as Danny's hair as being "bulletproof" while the two exchange constant insults about one another. Danny also developed a fear towards towards the NCIS: Office of Special Projects Manager Henrietta Lange, but he ultimately formed friendships with the whole OSP team and once the smallpox case is solved, he and Chin return to Hawaii.
In the finale, he fought Rachel for custody of Grace when it's revealed that she and Stan were planning to move to Las Vegas.
Danny met Steve's mother in the first episode and even informed Steve of Wo Fat breaking out of prison as well as the fact that Dr. Malia Waincroft, Chin's wife had died.
Following an appearance in family court, Danny gained custody of Grace.
In I Ka Wa Mamua (episode), it is revealed that his daughter was named after his his former partner who died after being beaten by two thugs who Danny later shot dead before escaping.
In the Season 3 finale episode, Aloha, Malama Pono (episode), Danny reignited his relationship with Gabby.
In Ina Paha (episode), in an alternate reality that Steve hallucinates, the Danny Williams of that reality who saved the alternate John McGarrett from being killed by that reality's Victor Hesse is more laid-back and carefree compared to the normal Danny.
He has also adjusted to living in Hawaii and even speaks the Hawaiian language.
In addition, he is still married to Rachel Edwards and is even seen wearing a wedding ring with the two's relationship being as strong as ever.
Despite that, Danny in the alternate reality is much more aggressive, almost ruthless when it comes to interrogating suspects, similar to Steve in the main timeline. This is shown when Danny interrogates the AU Victor Hesse by shooting him in the kneecap while using a pillow to muffle the gunshot.
Steve and Danny are partner in the field and also the one who is more than likely to give a headache given Steve's preferred method of doing things by himself. Their partnership began on rough grounds as Danny was the Detective assigned to investigate the death of Steve's father, John.
After some rough beginnings which ended with Danny punching Steve and telling Steve that he didn't like him, the two later got past their differences and began working alongside each other although Steve's actions at times such as going into the field without back-up manage to annoy Danny to no end.
Fans of the show have often compared Steve and Danny's friendship as a bromance between the two with the term McDanno being coined to suggest a possible romantic relationship.