Matthew Simmons

Matthew "Matt" Simmons is an agent of the FBI's International Response Team (IRT), which is featured in Criminal Minds: Beyond Borders. He and the rest of his team were introduced in Beyond Borders. He reappeared as a main character in Season Thirteen when he joins the BAU.

Background
Simmons was born in Michigan to a Caucasian father and a Korean mother. Because of his mixed ethnicity, his paternal grandparents ostracized him. In his adult life, he joined law enforcement in Detroit. He and his wife Kristy have been for te four children: two sons Jake and David, and twin daughters, Chloe and Lily; this fact has caused jokes about him being a "breeder". He is an FBI agent and specifically worked for Jack Garrett and the International Response Team. Through this job, Simmons has some prior history with Derek Morgan and Jennifer "JJ" Jareau. He is a military veteran who has seen combat overseas, presumably in the Middle East. This allowed him to hone his split-second decision-making skills. Hence, he is usually the quickest of the team to draw up a preliminary profile on the unsub.

Beyond Borders
Simmons is first seen coming out of the SUV with Jack and Lily Lambert, who are all on their way to the plane. There, they meet up with Morgan and JJ, who learn about Matt's newborn twin daughters. They then board the plane and discuss the case during the flight to Barbados. During the investigation, Simmons is paired up with Morgan and JJ. He delivers the profile along with the other members of the BAU and the International Unit. Afterwards, they looked up on the computer and eventually identify the unsub as Jerry Tidwell. The following night, Simmons uses a speedboat to get to Tidwell's boat, where he is holding the Sullivan family captive. After Tidwell is killed and he is informed about the son Nick being thrown overboard, Simmons locates Nick on a small raft, rescues him, and brings him back aboard Tidwell's boat. Back in Quantico, Simmons, along with the others, watch as the Sullivans are reunited with their relatives.

Spencer
Simmons reappears in the episode, working alongside Clara Seger when they are called in by Prentiss to help Reid, who was arrested in Mexico for drug possession and the murder of Nadie Ramos. At the time, the IRT was investigating another case in Costa Rica. The two IRT agents arrive at the local jail Reid is being held in and try to stop his transfer to Prisión de Máxima Seguridad El Diablo. Simmons tells Reid about a new lead, adding that there is no contraband on him. When Reid asks what were inside the vials of experimental medication he had, Simmons gives him a list of the medication's ingredients. This helps the BAU pinpoint Nadie's hometown. Just then, the Mexican police are given orders to transfer Reid to El Diablo. However, the agents are able to stop the transfer and have him extradited back to the U.S. after they realize the victim has dual American-Mexican citizenship. Afterwards, Simmons is seen onboard the BAU's plane, thanking Rossi for having him and Seger flown back to Quantico.

Criminal Minds: Beyond Borders
Simmons is portrayed as the muscle of the team, similar to the role Morgan played on the original show.

With the BAU
Simmons returned as a main character in Season Thirteen. In Wheels Up, he was contacted by Garcia following an attack on the BAU and join the effort to rescue an abducted member of the team.