A story isn't a story without the people who fill your world. In your sourcebook, you most definitely need to hit upon those same people, because your readers are going to want to know more about them.
One of my chapters will be devoted to the characters of my book. Some time in the past I found a wonderful outline to help you flesh out your characters. I'm not sure where, but it was just enough to get the creative juices flowing. I figured I would use a similar format for my book, but make it flow just a bit better. Here are the highlights I suggest you throw in about your characters:
1) Name and a short biography. This really depends on when you make your companion book. If it's with your first book in the series, you don't want to put too much in there. If it's after the series is done, feel free to let it all hang out. I mean, if someone is picking up your companion book, they've most likely read your stories and know all the plot excitement already!
2) Add statistics, because followers of your stories love that sort of thing. Height, weight, race, age, how they look and what they wear. Even speaking style is good. Do they speak with an English accent or, like Krom, rolling r's?
3)Their best and worst qualities. Elly loves animals, is a quick thinker and loves good jokes. But she is obsessed with killing Wizards, finding out who she is and saving children. This gives your character a little more depth, even though some of these things should be obvious when you read the stories.
4) You can also add in where they are found or how they are integral to the story. Shadowdancer was just supposed to be a random waif that Krom uses to find some information but became an important cog later on in the series. It happens more often then you think!
5)Lastly, add in some random facts about your character. Elly knows how to weave, while Krom can carve. Small things such as this make the characters a little more alive.
Keep on writing and good luck!