[4] Little is known about his early career with the Honolulu Police Department except that he had been assigned to District 5 (in real life, covering the areas from Kalihi to Salt Lake) and that Steve's father John was his training officer.

In the season 2 finale cliffhanger, Chin is put in a difficult position after a group of corrupt ex-HPD cops simultaneously hold Kono hostage and threaten Malia in the couple's own house as part of a plot to free Frank Delano from Halawa.

To the members of the Five-0 task force and to the general public, Kono had been stripped of her badge by the Internal Affairs Department of the HPD, but it was later revealed to be a ploy for her to go undercover to bust a string of dirty ex-cops.

In a scene deleted from the Pilot episode, Steve mentions bailing her out of trouble more than once and keeping it from their father to "let Dad go to his grave believing that you were his perfect little girl".

In season 3 Mary returned to Hawaii working as a caretaker, originally not wanting to reconnect with Doris she later meets her after strong encouragement from Steve and the person she was caring after.

In the season 2 episode "Haʻalele", it is revealed that his biological mother, Machiyo Takeshita, was murdered by the notorious "Trashman", a serial killer in Hawaii from the 1980s who earned the nickname because his victims were all found in trash bags or the dumpster.

It is implied that they are in a relationship (as of Season 6 Episode 14) as she is mentioned by Dr. Bergman numerous times in conversations with other members of Five-0; Rumer Willis, who portrays Sabrina, has only appeared twice on the show.

[13] Max retired from the Honolulu Medical Examiner's Office and from Five-0 in the season 7 episode "Ua ho'i ka 'opua i Awalua" and returned to Africa as a member of Doctors Without Borders.

[15] In the season 4 episode "Makani ʻolu a holo malie/Fair Winds and Following Seas", Cath persuades McGarrett to go with her to Afghanistan to find a young boy named Najib.

She once again returned in the series seventh season for the shows 150th episode to inform Steve that his mother had been detained following an attempt to break Wo Fat's father out of prison.

She returns again in the eighth season's twentieth episode, getting Steve's help once again to find someone making dirty bombs out of an unused military bunker's depleted uranium.

He was often at loggerheads with McGarrett over what he felt was Five-0's tendency to be trigger-happy with armed suspects and refusal to obey a "wait for SWAT" order, to the point where he lodges an official complaint with Governor Denning.

In the season 4 finale, Grover's actions to rescue his daughter result in forced early retirement from HPD, but also allows McGarrett to recruit him to be part of the Five-0 team, which he accepts.

Unlike McGarrett, Danny Williams took longer to accept Grover but they bond over the fact that they are the only members of the team with children and would often vent their frustrations to one another about parenting issues.

[29] Members of the Five-0 Task Force and their families are often seen patronizing his shrimp truck or asking him for favors (e.g. he babysat Grace Williams, Danny's daughter, and chaperoned Steve's sister Mary while they were at work).

Junior had a sister Maya, who was killed in a car accident when he was young, and their father never fully got over it, leading to him vehemently opposing his only remaining child enlisting in the Navy.

Their relationship hits a rough patch when Junior decides forgive the man who was driving the car that killed Maya and his grief-stricken father disowns him in the season 9 finale as a result.

[39] John McGarrett was murdered by Victor Hesse on September 20, 2010, as witnessed by Steve, who was overseas on a highly classified mission in South Korea, via satellite phone.

Steve was hostile to his mother, even refusing to call her "Mom", due to the years of resentment over how the family was torn apart by her supposed death and watching his father grieve over her.

She is mentioned in the episode "Pono Kaulike / Justice for All" where she uses her contacts to keep her son safe from a rogue CIA agent who attempts to have Danny and Chin framed to cover his tracks.

In the episode "Ua Ola Loko I Ke Aloha / Love Gives Life Within" she returns to Hawaii with Leonard's ashes on the pretext of spending time with Steve, Mary, and Joan, but does not tell them that she was actually dying and trying to complete her bucket list.

In the episode "Poina ʻOle / Not Forgotten", Nahele was first introduced as a homeless juvenile delinquent who stole McGarrett's prized vintage Marquis and stripped it for scrap metal.

McGarrett decides not to file charges after learning that Nahele was living on the streets after running away from his temporary guardian, his negligent uncle, and that his only parent, his father, has been in prison since he was eight.

The two strike up a deal; Delano will be able to get stolen drugs off the island in exchange for giving Wo Fat the location of Shelburne, who happens to be Doris McGarrett, Steve's supposedly dead mother.

In "Aloha, Malama Pono", Wo Fat is seen lying in a bed in a high risk detention facility at an unknown location with Steve standing in the room, staring at him.

His appearances are often met with annoyance by the Five-0 team as he would mock McGarrett and Chin, try to rile Danny up or crack inappropriate jokes at Kono (he calls her "Spicy Hot").

August March (Ed Asner) is a renowned diamond smuggler who was jailed for over three decades and had been released from prison for six months, according to Captain Fryer in the season 2 episode "Kalele / Faith".

[20] Jason "JC" Dekker (Xzibit, credited by his real name Alvin Joiner) is a former gang leader and small-time arms dealer who was re-arrested for violating his parole.

He was initially housed in protective custody due to his former occupation but was sent to "general population" after he aided his brother Daryl in the trafficking of conflict diamonds, indirectly causing McGarrett's sister to be coerced into becoming a mule.

Jason Duclair (Randy Couture) is a serial arsonist and murderer who was notorious for picking married couples and setting their houses on fire, after ensuring that their doors, windows, and any exits were locked.