Mysteries of the Arcana began in its present form as Chrys's story. As I began developing it, however, I realized the story needed two things. First, it needed a love story. Second, it needed a point of view character.
An point of view character serves as a proxy for the reader. When a story universe has a complex history and rule system it is helpful to make one of the major characters an outsider. Someone to whom the universe has to be explained. Harry Potter's a perfect example of this. He knows nothing about the wizarding world and, so, it doesn't seem out of place when absolutely everything needs to be explained to him. If Harry were raised with a full wizard family, it would feel stilted and awkward if someone explained Quiddich to him. Since Harry is an outsider he needs to be educated and as a result the reader is educated as well.
Having decided to introduce a love interest/point of view character, I began brainstorming. I didn't need to do so for long. Theresa, like Athena from Zeus's head, emerged fully formed. Short dark hair. Army jacket. Love of guns. Somewhat short tempered personality. Deciding she was Catholic came after realizing that love at first sight was boring. There needed to be struggle and conflict along the road for Theresa and Chrys's relationship.
I also knew I wanted to start off with her marching to her doom and finding a reason to live. The reason for Theresa's desire to kill herself evolved over time but was firmly in place by the time the comic debuted. I'm very glad. In a lot of ways, Theresa has overtaken Chrys as the "star" of Mysteries of the Arcana. She's a great character.