|Real Name:||William Harrington|
|Title:||Lieutenant Commander William Harrington|
|Interests:||Catherine Rollins (Ex)|
|Parents:||Tom Harrington (father)|
|Affiliations:|| USNR (Retired)|
|First appearance:||Aloha, Malama Pono|
|Portrayed by:||Justin Bruening|
Billy Harrington was a former Lieutenant Commander in the United States Navy and also the ex-boyfriend of former Navy Lieutenant Catherine Rollins.
Season 3 Edit
In Aloha, Malama Pono, Billy is seen at Joint Base Pearl Harbor-Hickam. He spots Catherine on the phone and waits for her to finish speaking. She smiles and hugs him, Billy tells her that he is retiring from the Navy. Catherine asks him what he'll be doing with his next, and he offers to tell her over dinner. Catherine sadly says that she's seeing someone at the moment. Billy correctly guesses that she is with Steve, and says that he thought that was over. Catherine says things change. Billy then comments, if things change again that he'll staying at the Ilikai before hugging Catherine again and parting ways.
Season 4 Edit
In A'ale Ma'a Wau, Billy enters Five-O Headquarters and is greeted by Steve with a hug. Steve mentioning it's been awhile since they're seen each other. Billy puts his foot in his mouth and says the last time he saw Steve was just before Steve was reassigned to a classified op. This op was the one that ended with the death of Steve's dad. Billy immediately regrets opening his mouth and tries to apologize, but Steve tells him not to worry.
Billy then says the reason he dropped by was to ask Steve something and thought was best that he do it in person. Steve takes him into his office and asks what he can do. Billy tells Steve that he's left the Navy, Steve congratulates him and asks what he's doing for work. Billy says that's the rest he's here, he's starting a private security firm and he's offered Catherine a job running his Intel Department.
Steve says that would be a great opportunity for Catherine if she was leaving the Navy. Billy immediately realizes that Catherine hasn't told Steve her plans yet and he apologizes for opening his big mouth. Steve stops him and asks him if she took the job. Billy says she turning him down. Steve asks is she said why and Billy replies no, but he suspects that working with him might affect her relationship with Steve. Steve says he knows that Catherine and Billy had a thing and Billy says that he'd never do anything to get between her and Steve.
Billy says he's only there because Catherine is the best for the job and Steve agrees saying she would the perfect asset to any team. Steve says he'll talk to Catherine before asking if everything between him and Billy are good. Billy says yes, shakes Steve's hand and Steve walks him out of his office.
In Ka 'oia'i'o ma loko, Billy surprises Catherine at Naval Intelligence with a retirement cake and her friends. When Catherine tells him he didn't have to do this, he replies he was a bit worried that she would have second thoughts. Catherine tells him that the job is all she could ever ask for. Billy says that's what he wanted to hear. Everyone cheers as he says it's time to eat the cake.
Billy is later seen at Catherine's retirement ceremony.
In A ia la akuIs, Billy is outside Five-O Headquarters waiting for Catherine to return from inside. Steve and Danny pull up next to him and Steve wonders why he's there. Steve introduces Danny to Billy. Billy says he's glad he ran into Steve as he never got a chance to thank him. Steve says for what. Billy replies, with Steve being cool with Catherine working with him. Billy says Catherine told him that Steve encouraged her to take the job and he's lucky to have her and they make a great team. Steve tells not to mention it and shakes his hand before going inside headquarters.
Billy then contacts Catherine through their earpieces, using code names, to let her know that Steve is heading to her position. Danny says it's cute that they have code names for each other.
Billy drops Catherine at Steve’s home after work. In the car he says that part of him had doubts they could make it work, after seeing Catherine in action, he knows he made the right choice. Catherine thanks him, but says it sounds like he's congratulating himself. Billy says it kind of did, didn't it, causing Catherine to laugh. Catherine exits the car and Billy says he'll pick her up at 8 the next morning. Catherine says coffee and malasadas are on him and Billy agrees. Billy watches wistfully as Catherine walks up to the house.
Billy is still in his car as Catherine is greeted by Steve. He can't hear what's being said but he does see Catherine and Steve kiss. As Catherine goes inside, Steve notices that Billy is still there. Billy quickly starts his car up, and waves to Steve as he backs out of the driveway.
In Kupu'eu, Billy is at Makapu'u Point with Catherine. They're on a job, running surveillance from a van on a client's husband - John Cutler. Catherine is watching Culter and a woman make out through with a window with binoculars while Billy listens to the audio. Catherine says she's lost visual when the couple tumble onto the bed, Billy says luckily the audio's alive and well. Catherine says she doesn't think their client is going to need this much evidence of her husband's infidelity. She says a simple photo of the couple kissing would convince their client. Billy replies that they were paid to get the true and that's what they're going to do.
Catherine asks how long the couple have been in the room, to which Billy replies, 3 very long hours. Catherine complains that her butt is numb, and Billy counters that he's drunk 4 bottles of coconut water. Catherine says she warned him, and suggests he could use one of the empty bottles if he really needs to go. Billy comments that he's a trained SEAL and he can hold it.
Billy then points out that it looks like the couple are finishing, which Catherine is happy about. She wants to go home and shower, and maybe bleach her brain on what they saw. Billy asks if she going to quit on him. Catherine is shocked and asks where that come from. Billy replies that when he offered her this job he promised her an exciting post-Navy life, world travel, intelligence gathering. Catherine points out that last week they protected a foreign diplomat, Billy counters and asks this week she a peeping-tom. Catherine just says it practice of her surveillance skills.
As the woman leaves, Billy says they just wait for the husband to leave before he takes her home. He asks her before he drops her off does she want to get some pancakes, which Catherine says sounds good.
Before they can leave a new car pulls up to the house. Catherine pulls out her binoculars and watches another man enter the house. She notices the man has a gun and alerts Billy. Catherine and Billy both get their guns and exit the van. They hear gunshots and make their way closer to the house. Billy yells for the man to drop his weapon, but he opens fire on Billy causing him to duck behind some trees. The man pulls out an auto gun and start shooting at both Catherine and Billy as he tries to escape. The man manages to shoot Catherine in the arm, Billy asks if she's okay, she says she's fine. The man gets to his car and fires off a few more rounds, this time hitting Billy in the stomach and he goes down. Catherine gets back up and fires at the car tail lights as the man drives away, while making her way to the fallen down Billy. She tells him he's going to be fine as she open his jacket the see the damage. Billy struggles to stay awake, while Catherine tells him he's not going to die tonight.
Catherine has managed to get Billy into the van and drives him to the hospital, yelling at him to stay with her. Billy says he's not going anywhere and that she got the excitement he promised. Catherine says this is not what she had mind. Catherine drives up to the emergency entrance, honking the van horn loudly. As she pulls Billy out of the van, doctors and nurses come rushing out. She tells the doctor Billy's injuries and what blood type he is - O-neg - while helping into the gurney. The doctor notices that she's injured too, but Catherine says fine. She watches as Billy is wheeled into the hospital.
When Catherine wakes up in the hospital, she is told by Steve that Billy coded on the table and the doctors were unable to revive him.
Billy's body is later seen in the morgue, Joe is standing over his body. He tells the medical examiner that comes in just who Billy was. He says this man’s name is William Harrington, former lieutenant commander, SEAL team 7. He saw action in Iraq and Afghanistan, he served his country with courage and distinction. The medical examiner says he'll take good care of him. Joe says he believes that the examiner will, he just wanted him to know who Billy was.
A few days later, Billy is buried with full military honors, despite just retiring from the navy. Steve, Catherine, Danny, Chin and Joe all attend Billy's funeral and wake.
- Served in the Navy SEALs with Steve McGarrett. 
- Was part of SEAL team 7.  
- Saw action in Iraq and Afghanistan.  
- When working security detail with Catherine Rollins, Billy's code name was "Big Kahuna" 
- Unlike Steve McGarrett, Catherine Rollins and Billy both retired from the military, instead of joining the Reserves.   However, Billy still received a military funeral. 
|La O Na Makuahine||Kanalua||Lana I Ka Moana||Popilikia||Mohai||I Ka Wa Mamua||Ohuna||Wahine'inoloa||Ha'awe Make Loa||Huaka'I Kula||Kahu||Kapu|
|Olelo Ho'Opa'I Make||Hana I Wa 'Ia||Hookman||Kekoa||Pa'ani||Na Ki'i||Hoa Pili||Olelo Pa'a||Imi Loko Ka 'Uhanet||Ho'opio||He Welo 'oihana||Aloha, Malama Pono|
|Aloha ke kahi I ke kahi||A'ale Ma'a Wau||Ka 'oia'i'o ma loko||A ia la aku||Kupu'eu||Kupouli 'la||Ua Nalohia||Akanahe||Ha'uoli La Ho'omoaika'i||Ho'onani Makuakane||Pukana|
|O kela me keia manawa||Hana Lokomaika'i||Na Hala a ka makua||Pale'la||Hoku Welowelo||Ma lalo o ka ili||Ho'i Hou||Ku I Ka Pili Koko||Pe'epe'e Kanaka||Makani 'Olu a Holo Malie||O ka Pili'Ohana ka 'Oit|