I personally think Sora shows some character development throughout the series.
In Kingdom Hearts, Sora is just a simple islander who has a crush on Kairi and a rivalry with Riku. Although the pair are friends, Sora seems to show some animosity towards him. He's bigger, stronger, seems more skilled in everything, and he teases about sharing a
Paopu fruit with Kairi. He's just a young kid experiencing childish emotions. However, when the island falls apart, Riku leaves, and a Keyblade is placed in Sora's hand; he gains a new sense of responsibility and emotional depth to him, realizing what in his life matters to him, and what he needs to do to protect it.
As he journeys throughout the worlds, he realizes what true friendship is all about, what it's like to see a loved one change, and what it's like to be separated from a loved on all the same. He wants Kairi and Riku to be back, and he wants to change Riku back to his old self (void from darkness). Think of it as an older brother growing up and changing for the worst. That's Riku to Sora, and Kingdom Hearts shows how he deals with this, and how he tries to change him back. However, it also teaches how you cannot always change people, and Sora learns this first hand. Through this, he meets new friends who he can relate with: which is another aspect of life that a lot of us have to deal with. Sora is no different, and it shows in his character development.
In Kingdom Hearts Re: Chain of Memories, it shows a new side to Sora. It shows how he matured a little (puberty voice
) from the past game. As he ventures through Castle Oblivion, he loses his memories. As he loses his memories, new ones are shown through his actions and the way he deals with them. Sora shows signs that he can still get sad, grow angry, be regretful, and other emotions that people thought he didn't have (thinking he was one sided, and cheery all the time.) It shows how a person can pretend to care for you and immediately become your everything (Namine), and try to change your life. That person tries to make herself seem like she's the only thing that matters, and in turn, forgetting your old friends, and the people that really mattered to you. Again, I'm sure a lot of people have been in situations like this before, and Sora is no different (although his is more adventurous). At one point, he even completely abandons Donald and Goofy, some of his two best friends. Sora learns though that they really are his true friends though, once they come to save him later no matter what, which teaches him to keep your good friends close, and not let others get in the way. Your friends tend to always know best. In the end, he even learns to forgive, saving Namine by own choice, and hearing her out. He doesn't let his emotions get hold of him.
In Kingdom Hearts II, Sora matures physically and mentally. These changes show in the game, affecting his actions. He shows a sense of empathy towards the Nobodies, wishing he didn't have to strike them down. (This is also shown sometimes in CoM, but KHII more so.) This shows Sora is able to actually think about who his enemies are, instead of just listening to people influencing him. He makes his own opinion on a Nobody or person, and then decides if it's right to fight them. He learns that there's all different kinds of people in the world with different goals. He learns the struggles of people. (Tron, the Nobodies, the Final Fantasy characters wanting to go home.) Sora learns a lot.
To say Sora shows no character development astounds me. He learns a lot about how to act, and what to do when situations arise. He learns a lot about his own emotions and how he affects people around him. And, this doesn't all happen at once: he develops through the series as a whole.