Colin is the younger of two brothers within the gang. In actual fact he has another brother, Jason, who is three years his junior and who makes a brief appearance early into the story, but he is too young to have been a part of the experience and thus doesn't really feature.
But Colin is not just one of the founder members of the gang. He is quite central to the story if one looks at the gang as a fishing net, with its interwoven cords, rather than as a spider's web with all threads leading to and from the epicentre. In The Best Year Of Our Lives Colin is a boyfriend to Beverley and a brother to Jim, with whom he is fiercely competitive for Paul's respect, and indeed for the respect of others.
He is one of the four members of the eight who begin the story as first-years at their respective primary schools, but he is tall and strong and looks older than his eleven years. He lacks Jim's maturity but they are close enough to be competitive.
Loyalty is Colin's watchword - to Paul, to the eight, and in a sometimes less than obvious way to his big brother also. Most of all he is loyal to Beverley, which at their tender ages is no small achievement. Paul at one point in the story finds himself reflecting upon what might make two kids of their tender vintage an item as opposed to just friends, but whatever it is Colin and Beverley have it and retain it (more or less) throughout.