Bradley Jacks is a Lieutenant for SEAL Team 9 who made his first appearance in Kame'e where he fell victim to an assassin hired by a Central American drug cartel, Hector Ruiz, who poisoned him before throwing Jacks out of the plane, obviously hoping to disguise the death as an accident or suicide.
Luckily though, Hawaii Five-0 team leader and Navy SEAL, Lieutenant Commander Steve McGarrett, who had caught on the scheme, sky-dived and after a failed attempt, he was able to connect Jacks' parachute to his own, enabling the two men to land safety to the ground while Hector was later killed by Detective Danny Williams and Lieutenant Chin Ho Kelly.
Jacks later reappeared in Kil'ilua where he was a member of the same undercover team including Danny, Kono and Chin that sneaked into North Korea to rescue Steve from Wo Fat and his men. Jacks having joined the team in order to repay Steve for saving his own life from near-death in Hawaii.