Emma's ex-husband, Madison's father, an animal behavior and communication specialist who formerly worked for Monarch, and co-inventor of "the Orca", a machine that enables communication with the Titans but can also "potentially control them using their bioacoustics on a sonar level.". After suffering a loss, Mark has an unfavorable opinion of the Titans, Godzilla in particular. Despite this, he joins a rescue mission along with Dr. Serizawa and Dr. Graham to save Emma and Madison from nefarious forces. Chandler spoke of the film's themes being about "healing the planet".

Mark's ex-wife, Madison's mother, a paleobiologist working for Monarch with a history of environmental activism, and co-inventor of the Orca. Unlike Mark, Emma believes that humans and Titans can co-exist peacefully. She and Madison are kidnapped by a mysterious organization interested in her technology, with their own plans for the creatures. Farmiga has described her character as being a "DJ for the monsters", stating, "She has figured out a way to communicate with the creatures..." She also described the film being about "saving the environment." Farmiga noted that while the previous film focused on a father/son relationship, King of the Monsters focuses on a mother/daughter relationship. Due to this, Farmiga believes the film may pass the Bechdel test.

Emma and Mark's 12-year-old daughter. Like her parents, Madison is academically gifted in science but she'd rather live a normal life. She has a distant relationship with her father since her parents divorced but she remains in contact with him. She is kidnapped alongside her mother. Lexi Rabe plays a young Madison under her credited name of "Alexandra Rachael Rabe."

A crypto-sonographer working for Monarch. Dougherty verified that Dr. Stanton is loosely modeled after Rick Sanchez from Rick and Morty. Dougherty had the character "drink a lot" to keep the character in line with the spirit of Rick Sanchez.

The wise and noble ruler of Agrabah who is eager to find a capable husband for his daughter Jasmine.

A paleozoologist working for Monarch as Serizawa's right-hand woman. Hawkins reprises her role from Godzilla (2014).

A former British Army colonel and MI-6 agent who defected after becoming disillusioned by humanity during his time in the service. Obsessed with "leveling the global playing field" and restoring the natural order, he became the mercenary leader of an anarchist eco-terrorist group funded through the trafficking of Titan DNA. In 2005, he was jailed in a Pakistani prison after encountering Monarch agents when attempting to breach the walls of an underground MUTO dig site with a band of accomplice mercenaries. After previously stalking Emma and attempting to raid some of her labs around the world, he takes her and Madison hostage to gain control of the Orca and further his agenda.[13][18] Dougherty described Jonah as a mysterious character with conflicting ideas about the Titans' role in the world, believing that mankind has damaged the planet and bringing back the Titans will potentially set things right.

Monarch's Director of Technology, leader on the development of the Monarch Sciences network, and communications liaison with the U.S. government.

A decorated colonel who formerly served as an instructor in the 75th U.S. Army Ranger Regiment and is now the leader of the G-Team, the special military forces group run by Monarch specializing in battles involving Titans.

A chief warrant officer who is a member of the G-Team.

An Admiral in the 7th Fleet of the United States Navy. He is the commander of the U.S. Navy task force and was previously in charge of tracking down the MUTOs. Strathairn reprises his role from Godzilla (2014).

A high-ranking scientist working for Monarch. His father Eiji Serizawa was one of Monarch's founding fathers, so he's seen by many as the agency's de facto leader. Watanabe reprises his role from Godzilla (2014). Watanabe noted the film's themes, stating, "In the 21st century we need to do think about natural disasters. This creature is symbolic of that natural disaster. We cannot control them, but we must live on this planet."

A mythologist working for Monarch, specializing in deciphering the mythological backgrounds of the Titans in connection with tales and legends throughout history. With her twin sister Dr. Ling, Ilene is a third generation Monarch operative in her family, having joined the agency like their grandmother and mother before them.