Her full name was Theodora but she only went by Theo. She had to have been born between 1910-1920. I always thought it was odd, even as a kid. I prefer Thea.
Ooh! I know a Theodora who also only goes by Theo (though her dad still uses the full version!) and she must have been born in about 1990. She despises both Theodora and Dora, though she's pleasant and pretty enough to carry anything off - she's an airline cabin attendant.