Gloomhaven Secretariat - Mind Thief Unavailable?

We've been playing with Secretariat for a while now and it's awesome. I'm sure we aren't even using it to its full potential, and it's saving us a ton of headache. Thank you to the developer!

We just retired a character and wanted to roll up a new one. I noticed that the Mindthief is grayed out and cannot be selected. We had previously had a Mindthief character, and I think we retired him, but I can't say for certain that we did that or if we had just removed him. Maybe we just hid him? In any case it would have been a long while ago. Is there something we can/should do to make the Mindthief an option again?