Hell, that's how Chris and Ami came to be in the first place. In general I agree with Neshomeh. Read as many spinoffs as you like, see what you're into/works really well, and go from there.

(Also, shameless self-plug here--you can check out all of my missions over on my Tumblr.)

Knowing what your characters are in for and seeing how others have written their own agents will give you some ideas about what works generally, what interests you specifically, and what doesn't.

Taking a look at the Permission article, if you haven't in a while, may give you some basic ideas about what we like to see in PPC agents. You could use some of the prompts just to practice and get a feel for your characters before you jump back into mission-writing.

Browsing through the various FAQ and Help articles on the wiki (second tab in blue top navbar) may provide some useful tips, too.

Also, I recommend PoorCynic's series of workshops, the first two of which are about creating characters and writing speech.

I hope some or all of that helps!

~Neshomeh