Sam is the 'human' controlled by the player in Akintale. It has no personality nor thoughts of it's own and has the same sex as the player. This means that it's gender changes (which means they are by definition not genderless) depending on who is playing, this can happen at literally any time if the keyboard is passed over to another player. The colour of the Sam's eyes change colour depending on who is in control, blue when a player. The body is controlled by a strings connected to the hands, feet and the top of their head. As they are a purely a player character, they don't really have a surname but 'Pawn' is used.
In a TP route, control is passed across to Frisk. The body's sex remains locked to whatever the player was even after Frisk gains control. The bodies eyes become green when Frisk is in control.
When on a Genocide run, Chara never actually gains control over the body, they kill Sans, Asgore and Flowey with their own inhuman form. They only gain control AFTER the player gives them their soul. The eyes are red/brown when Chara is in charge, depending on their mood. Chara cuts the strings after a Geno, the body is destroyed with the world.
When a neutral route is achieved, Sam is destroyed by the barrier as they are no longer needed. They never actually leave. This is why Sans judges them on every route except a TP and Geno.