Sergeant John McGarrett, HPD (Ret.) (1942-2010) is the father of Steve and Mary Ann McGarrett and (presumed) widower of Doris McGarrett. His own father, Ensign Steve McGarrett, was killed in action on the U.S.S. Arizona (BB-39) during the attack on Pearl Harbor and John was born three months later on March 15. John would follow in his father's footsteps into the navy, serving in the Vietnam War and reaching the rank of Lieutenant before becoming a Honolulu Police Department detective. After Doris' presumed death, he launched his own investigation and sent Steve, then a high school junior, to board at the Army and Navy Academy in Carlsbad, California and Mary to live with his older sister Deb, who lived several hours away from Steve's new school, for their safety. It is presumed that this was the last time the siblings ever saw him in person as they became estranged from their father for sending them away; Steve only learns the real reason why his father split up the family when he returns to Hawaii to head the new task force following John's murder. In light of the revelation, Steve and Mary were able to let go of their animosity and are often seen visiting his grave site.
Lieutenant Commander Joe White, USN (Ret.) (born July 9, 1954) is Steve McGarrett's former SEAL trainer and a long-time friend of John McGarrett and mentor to Steve. A Wisconsin native, he is a "mustang", having been a master chief petty officer before gaining a commission and retiring as a lieutenant commander. He appears to be well known amongst SEALs from his time as a trainer at the Naval Special Warfare Center, as McGarrett's former SEAL teammate Billy Harrington and SO2 Graham Wilson, who was a person of interest in the episode "Hoʻapono", both knew of him. In the season 2 episode "Kiʻilua / Deceiver", he is forced into retirement after leading an unsanctioned mission into North Korea to rescue McGarrett.
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 in the 1980s who earned the nickname because his victims were all found in trash bags or the dumpster. He was born to a Japanese-American mother but adopted by a Jewish family, the Bergmans, after a short stint in foster care, hence his last name. As an undergraduate at Arizona State University, he was a self-confessed party boy and earned the nickname "Beerman".
Eric Russo (born 1984 or 1985) is the nephew of Detective Danny Williams; his mother Stella is one of Danny's younger sisters. He was first introduced in the episode "Kapu / Forbidden" when Stella sends him to his "Uncle D" in hopes that Danny would be able to set him straight. He dropped out of college and was still undecided about his Future. In the episode, he follows McGarrett and the team around as they solve a case involving a college professor and his students. Eric provides some valuable insights which helped solve the case. To Danny's surprise, he decides to pursue forensics and joins the forensics lab as an assistant.
A 25-year veteran of the force, Grover graduated from the Chicago Police Academy in 1989, and was promptly recruited by the FBI to go undercover in the Philadelphia Black Mafia. Upon the completion of the assignment, he returned to the Chicago Police Department. He left Chicago around 2012 or 2013, after an incident where he blames himself for having failed to save a boy who was taken hostage by his father, which ultimately resulted in a murder-suicide.
Danny began his career as a police officer with the Newark PD. When he was a patrol officer, he testified against his training officer, Rick Peterson (Peter Greene), under a subpoena during Peterson's trial for corruption. He worked his way up to become a respected homicide detective with 87 convictions before transferring to the Honolulu Police Department. In 2001, he witnessed the murder of his partner Grace Tilwell by a pair of Gangsters they had been sent to apprehend. After barely making it out alive, he notices smoke billowing from the World Trade Center and was later ordered to New York City to assist the NYPD. That day still haunts Danny and his daughter Grace was named after Tilwell. He often describes his time with the Newark PD as a "happy time" of his life as he was still happily married to Rachel and Grace was born. Danny is portrayed as a loyal cop who is married to his job first and life second. As a consequence, he is often at odds with his ex-wife Rachel for sometimes not being on time to pick up Grace. He is very loyal to his friends and colleagues, once refusing to believe that his former HPD partner Meka was corrupt even when pitted against overwhelming evidence after the partner was found dead under suspicious circumstances. Likewise, he often gives Steve the benefit of the doubt even when the latter resorts to more "unorthodox" methods in an investigation as he trusts Steve's judgment.
Danny's pistol is an H&K P30 although he carried a SIG Sauer SP2009 in the pilot.
Jenna Kaye is an ex-CIA analyst, assistant to Five-0. She is later revealed to be an associate of Wo Fat. At the end of "Ha'i'ole", she is seen driving Wo Fat away from the prison after he murdered Victor Hesse. She leaves Five-0 to follow up on a lead that her fiancé, Josh, who she claimed had been killed by Wo Fat, might in fact be alive and asks Steve to go with her to North Korea to get him. It turns out to be a trap for McGarrett, from Wo Fat telling Jenna he would release Josh if she brought him McGarrett, explaining her association with him. Jenna later discovers that Josh had been dead the entire time and that it was also a trap for her. She is able to tell McGarrett she is sorry for setting him up and gives him a pin from Josh's knee repair to help him escape, moments before Wo Fat shoots and kills her. In the 100th episode she is seen in the same episode that Victor Hesse is in looking for her fiance who was in a motorcycle accident.
Adam Noshimuri, Kono's husband who becomes a new head of the Japanese Yakuza after Wo Fat murders his father, Hiro. First introduced in the season 2 episode "Pahele / Trap", he has been trying to legitimize and clean up his organization with the intention of distancing himself from his father's Criminal past. When he and Kono first started dating, the Five-0 team were extremely suspicious of him, particularly Steve and Chin – the former due to his father's history with the yakuza and the latter due to him seeing Kono as a younger sister. He is accepted into the Five-0 ohana after proving his love for Kono. After killing his brother Michael (Daniel Henney) to protect Kono, he and Kono leave Hawaii and go into hiding. He comes out of hiding after his brother's loyalists are no longer a threat. At the beginning of Season 5, Adam proposes to Kono and at the end of this season they marry. In "Mai ho`oni i ka wai lana mālie", Adam is shot by Gabriel Waincroft, who runs away with the money that Adam would use to distances himself from the Yakuza. Almost immediately after Adam killed two Yakuza thugs in "Piko Pau 'iole" to save himself and Kono, Gabriel buys off Adam's debt in order for his cartel to team up with the head of the Hawaiian Yakuza. He subsequently turns himself in to the Honolulu Police Department. After a short legal battle he is sentenced to 18 months in prison, which was agreed on since he had been acting in self-defense. He has been released on parole as of the season 7 episode "Ka 'Aelike / The Deal".
Debra "Deb" McGarrett is the late older sister of John McGarrett and aunt of Steve and Mary Ann McGarrett. She raised Mary when John had sent the kids away from Hawaii for their safety after Doris was presumed to be murdered by a car bomb. Steve and Mary both see her as a maternal figure and she, having never married or had children, treats them as her own. Deb had been a struggling singer on the cusp of a big break when she left a promising career to raise Mary. In season 4, she pays Steve a surprise visit for Thanksgiving and reveals that she had stage four cancer. In season 5, she agrees to go for chemotherapy and her brain tumor has shrunk. She marries Leonard Cassano (Frankie Valli), a retired defense Lawyer with stage four leukemia whom met during chemo. 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. She passes away peacefully in Steve and Mary's childhood home (which Steve has lived in since his father's death) after fulfilling all but one thing on her bucket list—climb a tall mountain. Steve and Mary complete her list by scattering her ashes on the mountain top.
Cath makes a surprise return in the season 5 finale for Kono and Adam's wedding. Steve welcomes her with a hug when she surprises him at his backyard. She remained for the first three episodes of the series sixth season however she leaves once again before Steve is able to propose, saying that she had something to do. A heartbroken Steve tells her that he could not wait for her any longer if she decides to leave again, and she departs in tears. However, she makes a call to an unidentified person stating that Steve believed her story and that she was "ready". 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 assists Steve and the rest of the Five-0 Task Force in rescuing Doris McGarrett and Wo Fat's father.
