Lieutenant Matias was an officer in the San Francisco Police Department and partnered with Detective Cheung.
Terminator Genisys timeline
Matias was partners with Detective Cheung and both were not fond of Detective O'Brien's various claims about suspects Sarah Connor and Kyle Reese. After the latter two escaped from the hospital with the Guardian, Matias helped lead a manhunt for all three protagonists.
Matias later interrogated Sarah on her various appearances at the various chaos that had erupted, still believing her to be a lethal terrorist. He was later killed by the T-3000, who was disguised as his partner Cheung along with two other federal agents. This sudden disruption caused the Guardian, who was being questioned in the nearby interrogation room by Detective Harding to break through the wall and defend Kyle and Sarah from the T-3000, who slowly got rid of its Detective Cheung appearance.