Detective Danny "Danno" Williams was a detective of the HPD or Honolulu Police Department. He was transferred from New Jersey in March of 2010 to Hawaii so that he could work in the same career and see his daughter. Steve McGarrett and Danny met eachother at an "active" crime scene, related to the investigation into the murder of Steve's father, Jonh McGarrett. After a brief and unfriendly confrontation with eachother, it wasn't long before Steve McGarrett made Danny into becoming his partner with his newly formed task force that was sanctioned by the Governor of Hawaii, from there they seemed to have grown a love-hate relationship with eachother. Steve enlisted Danny in the force because he's just what Steve was looking for.
Steve McGarrett calls Danny a "sensitive" man and makes a point of telling Danny that's the reason why his wife left him. Danny argued that he's "sensitive" only when he is shot, which was indirectly caused by Steve's impetuousness. Additionally, Steve likes to poke fun at Danny for his peculiar living environment; his feculant apartment. Steve also likes to make fun of Danny's nickname "Danno" which was given to him, Steve questions why he was given that nickname, Danny later replies that his daughter gave him the nickname, he further explains that his daughter was trying to pronounce his name but ultimately at the tender age of 3; all she could ennunciate was "Danno".
After Danny and Steve subdued the shiphands on the freighter, and Victor Hesse was shot and killed by Steve (at least they hope), Steve properly apologizes to Danny about him being shot on the job and offers him a 3-night stay at a hotel. This was another attempt of Steve to make sure Danny finds better living arrangements. Despite all of that, Danny and Steve did have a moment bonding after countless fighting with one another.
- Danno is the only member of Five-0 who does not originate from Hawaii.
- He has the ringtone set for his wife to the theme song of "Pyscho".
- Danno's name is part of an old catchphrase which originates from the original Hawaii-Five O, which is "Book 'em, Danno!"
- Seems to have a dislike for Hawaii, calling it a "pineapple infested hellhole".