Trevor is confident and strong-willed, and able to read others in order to manipulate them.
Trevor is described as having “brooding silence,” so in a diary entry you could explain what he is really thinking. For example, he could be nervous and just hiding it. Here is an exmaple:
When I first met the gang, I was intimidated by them. They knew that my family were outsiders, since my father had “come down in the world” and everyone thought my mother was putting on airs.
It does not take long before Trevor has become "T" and the other boys in the gang look to him more and more. Trevor seems to know exactly what to do to get the respect of the others.
They were ready to make fun of my name, so I made sure they used "T" instead of Trevor. I knew that "T" sounded cool and confident, and they would be more likely to listen to me and not tease.
This part of the entry explains why Trevor was able to ovecome his name. Other elements you could focus on would be why he suggested destroying the old man's house, why he took over the gang, and whether or not he knew what would happen.