One of two teenagers charged in the fatal beating of a man trying to break up a fight in Waikiki last year pleaded guilty yesterday to first-degree robbery and second-degree assault.

Benjamin Pada faces a mandatory 20-year prison term for the robbery and maximum five-year prison term for the assault when he is sentenced in April.

The charges stem from the fatal beating of 58-year-old Ned Nakoa Jr. in May 2008.

Pada was 18 at the time. Police said his co-defendant, Kelii Acasia Jr., who was 19 at the time, delivered the fatal assault.

Honolulu police said Pada and Acasia stole a purse from a couple on the beach at Waikiki. Pada hit the male victim, a Marine stationed at Marine Corps Base Hawaii, in the head with a wrench, causing a cut that later required stitches.

The Marine ran after Pada and punched him in the mouth, which caused the teenager to release the purse, police said.

Nakoa then intervened to break up a fight between the male victim and Acasia. Police said Acasia punched Nakoa, causing him to fall to his knees. Acasia then kicked Nakoa in the face, police said.

An ambulance took Nakoa to the Queen's Medical Center, where he died the following day.

Acasia is charged with second-degree murder. He is scheduled to stand trial later this month.