We figure out all her likes and dislikes. We flesh her out, maybe draw a moon or a tree or jail bars, give her a locale. I explain to the students that everybody has a secret power. Some people can be invisible. They can sit all day in a room, and nobody ever even notices them.
Some people can be in a really bad situation, and their minds can fly away to Tahiti.
I tell them of the thousands of tiny invisible shields that I sport daily, and how sometimes they're uncomfortable but it's better than getting hurt. The little ones sometimes ask if it's true. I smile, suppressing a shriek. Next, we give our character her power, and I tell the students to start thinking hard about their own powers. You can have a jillion.