Geno is Errors older brother, they both wake up in a pair fighting arena(When 2 people go into the arena, and one advances unless its a tie or the both opponents survive)by their Gaster, who is unknowingly the twos father, Error is 15 and Geno is 18, You start fighting at age 15 and the arena is NOT Merciful, however, if someone is unconscious, or incapacitated the arena will take you to get patched up and call it a win for your opponent