This section applies to the IALCOTN.
Charmander is, well, a Charmander. He was originally one of Ash Ketchum's volunteer starter Pokémon and was given to a boy named Taylor on his tenth birthday rather than at the official starter Pokémon giveaway. Taylor then cheated a girl to trade a higher-leveled Quilava for him (lying that the reason Charmander had not evolved was just that Charmander evolve a little later than Cyndaquil), and when the girl realized she had been tricked, she tossed the Pokéball to the ground in frustration and left Charmander alone in the rain on Route 201.
Charmander was then found by Mark after fainting on the road outside his house, and has agreed to be his Pokémon after the unlucky experience with his first two trainers.
To be a starter Pokémon, you need a certain mindset, and Charmander is no exception. You need to have a certain faith and trust in humans that will not be ruined by one person's beginner's mistakes; an open mind toward the many careers that the trainer may pursue; and as the saying goes, "A child becomes a man only by the combined efforts of his parents, his growth and his starter."
Charmander is a calm, forgiving personality with a sense of humour. He thinks of Mark half as a friend and half (somewhat jokingly) as a project to work on to make something out of him.