Frank Simpson was the abusive ex-husband of Detective Danny Williams's girlfriend Amber Vitale (real name Melissa Armstrong-Simpson).
According to Amber, she met Frank at 19, when she fell in love with him, as he was the first man who made her feel special. However, after they got married the next year, everything suddenly changed, as he became too controlling, and abusive when Amber stood up for herself. Twice, he landed Amber in hospital.
After he once almost killed her in 2013, Amber filed a complaint against him and obtained an order of protection against him, after which NYPD arrested him for felony assault.
In episode Nanahu, Frank arrives in Hawaii, just as Danny and Amber have gone on a romantic getaway. He first visits Kamekona's, where he shows Kamekona a picture of himself and Amber from their married days, seeking to acquire information. However, Kamekona passes the information over to Steve.
Later, Five-0 task force finds out about him, just as he moves to kill Danny and reclaim Amber. He ends up stabbing Danny, and threatens Amber at knifepoint. Later, Amber runs him over with Danny's Camaro, killing him.