Sam Stone (Serious Sam)

Sam was born and raised in the United States sometime during the 22nd century, though not much is known about Sam's past. At some point in his life, he attended a high school, and was nicknamed "Hairy" by his friends; this nickname apparently had something to do with goats, according to a conversation he had with Quinn in Serious Sam 3: BFE. His left foot is bigger than his right foot, and he also has a birthmark in a "very delicate spot". He is also a fan of bananas, as mentioned multiple times in Serious Sam 2. During the Mental War, Sam joined the Earth Defense Force and rose up through the ranks until he was promoted to Captain. In the early days of the war, he commandeered the Battlecruiser Saratoga, which he used to travel to many planets throughout the galaxy to convince the native species to join in the fight against Mental. When Mental's forces invaded Earth, Sam fought against the alien army, and, like many members of the Earth Defense Force, rallied with them to defend his planet.

Croteam Head (non canon apperance)

Bigheads appear as skinny humanoids with pale skin and large heads (an enlarged cranium or exposed brain) that has a face of one of the members of Croteam. They appear to wear the same clothes as Sam.

NETRICSA (NEuro-TRonically Implanted Combat Situation Analyser)

NETRICSA is advanced AI technology used by soldiers in the Earth Defense Force. She is a miniature computer, implanted under or inside the skull of her carrier. NETRICSA is connected to the human cerebral cortex, so she has all the access to the user's audio-visual experiences and can provide them with feedback. It is used to provide fast and simple access to all the necessary data and analysis for a skilled combatant, so that he/she can keep up to date with the situation and know what to do next.

Mental

Mental is the last of the Hum-Tah (the Infinite Ones), a species of god-like beings that once ruled over galaxies. It is suggested that Mental wipes the galaxy clean of intelligent life once every one hundred thousand years. In between these cycles of war, he enters a state of hibernation and supposedly leaves autonomous machines (mostly around homeworlds of extinct species) on guard, watching out for any activity in the galaxy.