Matthew "Matt" Williams (died 2014) was the younger brother of Danny Williams, uncle to Grace Williams and Charles Williams and a man who appeared in Loa Aloha (episode). He appears to be friendly with his brother's ex, Rachel Edwards. He is a stock broker in New York.
He traveled to Hawaii on vacation but his brother later learned he was suspected by the FBI of fraud. Danny reluctantly tips him off, however it turns out to be true - Matt had lost $58,000,000 in one day, so he took money from other clients' accounts, and to pay them back, he has been laundering money to the third most powerful drug cartel in Colombia.
He is able to escape the island because Danny couldn't shoot him. Grace really loved Matt and referred to her Uncle Matty as "the coolest ever".
Conversations between the two indicate Matt went to Danny to bail him out of trouble after crashing their father's car into a tree as a teenager. Also, when Danny was nine, he once handcuffed Matt with plastic handcuffs to a monkey cage at the zoo for causing a disturbance.
In another conversation between Danny and Steve, the detective implies that Matt once talked him out of committing suicide shortly after Danny's divorce.
Matt has not made any appearances since "Loa Aloha".
At the beginning of Season 5, it was indicated that the Colombian Drug Cartel had Matt hostage and demanded $18.5 million from Danny or else they will kill him.
After retrieving the money, Steve and Danny travel to Colombia to give the money but they soon find out that it was all a cruel setup, and that Matt had been dead and stored in a metal barrel the entire time.