Being sort of an "extra" for Team Pika, Charmander is one of the simpler, easy to understand characters in the series, and part of that may come from the fact that he's also just a child. Charmander is extremely cautious and seemingly afraid of everything. Whenever there's an unknown, Charmander tends to assume the worst, probably as a method of self preservation. Perhaps his fear of the unknown comes from a sense of abandonment? Afterall, he was left all alone at a very young age, and that can have all sorts of undesired effects.
There is something very interesting about Charmander, though, and that's his ability to cut through the bull and tell his team members straight up if there's something he doesn't like about someone or about a situation. He has a way of sniffing out untrustworthy people, traps, dangerous situations, and other such things. In fact, he's so in tune that he can also sense paranormal and even extraterrestrial activity. Early on in the series, the rest of the team thought Charmander either had an overactive imagination or was being too negative, but as the series progressed, and considering he seemed to have a pretty high average of being correct all things considered, the team began to trust Charmander's gut, no matter how crazy it seemed.
He and Nidorina share a special bond. When Nidorina, who is deathly afraid of extraterrestrials, had an encounter with one, the only person to believe her was Charmander, as he believed it was possible one could be lurking about in the woods. It wasn't until a reconnaissance and research ship had arrived that the other members of the Poké Forest believed them, but at that moment, Charmander and Nidorina would always share that bond of being suspicious about all things otherworldly. Charmander and Nidorina also share common ground in that they are both big eaters. Nidorina loves to cook for Charmander, because she knows he'll appreciate the feasts she prepares just as much as she will. And just like Nidorina, Charmander never gains a pound! They love to talk about all the wonderful things they'd love to try to eat, and can spend hours watching cooking shows or looking at recipes online.
Charmander is also friends with all of the children in the forest. Despite his cautiousness, the other kids really seem to like him for his tendency to be honest about himself, and for being very true, helpful and loyal to others (things he's learned from being with Team Pika). And–perhaps they'd never really say it out loud–the kids also like listening to Charmander's exaggerated stories of events that have happened to him and the rest of Team Pika. No matter how bad these events actually were, Charmander has a way of spinning it to make it sound about 11 times worse. He's pretty much unrivaled at telling spooky campfire stories, and he does it without trying! For all he knows he's just recalling the events as he saw them!