Teodora Kosara is a Vesper agent, who formerly held the title of Vesper Six. Teodora is a former member of the Cahill Family, an Ekaterina by birth, like her relative, Ana Kosara. On the back of her card, she seems to be a troublemaker. She resides in Bulgaria.
Black Book of Buried Secrets
"Growing up in Bulgaria, Teodora didn't have access to the same resources as young agents in real Ekat hot spots around the world. However that didn't stop her from devoting herself from the Clue hunt, using every opportunity to conduct research and follow leads. While on a class trip to London, she sneaked off to explore the Tower of London, a known Lucian stronghold. Teodora was obsessed with the 39 Clues but her allegiance with the Ekaterina is questionable. She has ties with the Vespers--the only other group in the world who knows about the Clues."
Vesper Six used the name Teodora while posing as a skydiving teacher for Amy Cahill and several other Cahills, and attempted to steal Gideon's Ring from her. It is unknown if this person was Teodora Kosara or another Vesper, Cheyenne Wyoming, who held the title of Vesper Six and who matches the description given better.
Her card is Card 169.