Mason Hewitt is a sophomore at Beacon Hills High School in Season 4. He becomes Liam Dunbar's best friend after the latter transfers. Mason is kept in the dark when Liam is unintentionally transformed into a werewolf by Scott McCall, but he remains supportive of Liam and comes to meet Scott's Pack. Mason is then attacked alongside Lydia by a Berserker, being fully exposed to the supernatural world and he is eventually let into the supernatural circle of Beacon Hills by Liam and the Pack in Season 5. Mason helps the Pack during their fight against the Dread Doctors and the chaos caused by the Chimeras. He and the Chameleon Chimera Corey Bryant develop a romantic interest in each other. He helps Corey recover his humanity after the latter is brought back to life. Mason is revealed to have genetic chimerism because he'd absorbed his unborn twin in utero, making him a genetic chimera and is revealed to be the success for the Dread Doctors: a vessel with which to revive Sebastian Valet, the Beast of Gevaudan. Once Sebastien remembers his name, Mason is erased from existence. Lydia saves Mason's life using her banshee voice, Mason remembers his true self, his being separating from Sebastien's spirit. Mason, alive and well, moves with his life and Corey.
Mason is an immensely kind-hearted, loyal and supportive individual to a fault as he does his best to help his best friend Liam keep a tab on his rage issues serving as Liam's voice of reason, confidant and wingman , the way Stiles was to Scott.
Compassionate, non-judgemental and fair-minded, Mason is always happy to lend a hand to someone who needs it.
Despite his age and his occasional teenaged admiration of the occasional male physique, and then his boyish excitement over his introduction to the supernatural and meeting the local high school Pack, Mason is also quite mature and patient, being openly gay and honest on his opinions. He holds true to his morals when confronting Corey, his romantic interest over the latter's affiliation with Theo after the lead Chimera's unscrupulousness was revealed.
Mason is also quite bright, being intelligent and seems to be always curious over peculiar circumstances occuring in Beacon Hills, always willing to investigate a mystery. Despite the dangers present, and takes said threats seriously, Mason finds his new world fascinating, and gives the Pack assistance whenever they need it.
Mason notices Liam's strange behaviour when Liam runs three miles to get to school instead of getting the bus. He notices that Liam's leg has healed. Mason invites the entire freshman year to Lydia's lakehouse when Liam texts him about the 'party'.
Mason is seen working out with Liam in the school's gym. He tries to calm Liam down when Liam finds out that the lacrosse match is against Devonfort Prep, Liam's old school. At the game, Mason tells Liam that he can take Brett on. Mason admits to Liam that he thinks Brett is hot. They both laugh it off.
Mason notices Liam's strange behaviour over the Deadpool. At the annual bonfire, he tells Liam that he wants to no what going on with him, but he also wants to help Liam. When Scott, Liam and Malia are unwell at the bonfire, Scott tells Mason that it's the music affecting them. The assassins wired a sonic weapon into the music. It was weaking them because of their sense of hearing. The assassins, posing as security guards, take Malia, Liam and Scott away, Mason is concerned. He pulls the plug to the music player. He unknowingly saved Scott, Malia and Liam from burning to death.
Mason seems to be friendly enough with Scott that they have exchanged phone numbers. He tried to help Lydia the night of her party by insisting that they could get the wine stains out of the carpet when she got overly upset at the spill.