Plato has a couple of traits, such as loyalty, and compassion. He is loyal by staying with Jim, even though he has only known him for about a day. He goes to the "Chicky run," he goes off and finds him. He shows compassion, by when Jim is going to do the "Chicky run" he looks horrified by the idea.
Plato has grown up without his parents, both of the other characters have parents, though they don't get along with them, but Plato has no parents at all. He has grown up with his nanny, and he desperately longs for a father, and that is why he likes Jim so much. He sees him as his father, and Judy as his mother, so he was trying his best to live a fantasy with them.
Plato does not learn anything throughout this movie.
PLATO'S MISMATCHING SOCKS
Plato's mismatching socks could represent the chaos in his life. His life is very discombobulated and that could be represented by the socks. Another possibility is that he was in a rush, and had to hurry to get socks, and his room was dark, so he grabbed the first two socks he saw... just so happening to be blue, and red.
One of the main themes throughout the movie is manliness. Jim's father is always being told what to do by his wife, and mother, and Jim notices this. He sees how his father is easily "pushed-over" which agitates Jim greatly, even to the point of almost killing his own father. Another theme is loyalty. All of the friends show loyalty to each other, by not leaving one another when things were going badly. They have barely known each other for a day, but their loyal bond is already strong.
If Buzz would have lived to the end of the movie, do I think he would become a "good guy?" I think that it depends on how the chicky run went. If Buzz would have won the chicky run, I think that he would have still been a "bad guy." I think though that if he had lost to Jim during the chicky run then he would have respected Jim, and then become a "good guy."