Green debuts in the first round of the manga, A Glimpse of the Glow where he and his rival Red see a Mew in a Pallet Town forest. His first Pokémon was a Scyther -- he is shown with one when he was a child. When he was young, he trained in Johto under Chuck, the Gym Leader of Cianwood City.
Green has a serious, calm, and focused personality in everything he does. Although Red and Green were once bothered by each other because their personalities are so opposite, they ended up resolving their differences.
Green and his Scyther were trained by Chuck of Cianwood City when he was a child. Thus, Green has some cursory knowledge of martial arts, and his Scyther knew Generation II techniques.
Green also has a rivalry with Koga of Team Rocket. This began when Green almost killed Koga's Arbok by having his Charmeleon cut it in two, and continued on throughout the series until Koga's reclusion