Theo was only in, like, 3 episodes. I don't think he even constitutes a "supporting" character at that point. Sadly, if he was supposed to have an arc, it never really went anywhere either.
I agree, but the basis for his character in Star Trek would be rather enjoyable. I think his missionary group's purpose was to study the religious faiths and texts of the various alien races on the station. If the next Star Trek is based on a TOS/TNG "Explore New Worlds" format, having a character around to learn and understand the religious and spiritual customs of alien races would be rather interesting.