Spoilers: Kieran is a mystery until Yuuri's Hot Date (YHD).
AKA: Tomas Vespasian
Age: ~ 65 in year 0 (~70 in YHD)
Looks: ~5'2", a female twin to Wolfram.
Character: Tomboy, passionate and changeable, loyal and dutiful to Donaghie and her father Lord Donaghie. Unworldly and woefully undereducated for an aristocrat.
Family: born to Danielle von Donaghie and Dougal Lord Donaghie. Dougal suffers many illnesses due to exposure to pitchblende (uranium) in his youth. The couple had a number of stillborn and miscarried children with serious abnormalities. Manfred von Bielenfeld agreed to serve as a secret sperm donor - so Kieran is Wolfram's secret half-sister.
Danielle was Cecilie's first cousin. She died when Kieran was ~25. After that, Kieran stayed fairly isolated in Donagal, the Lord's seat of the domain of Donaghie, caring for her ill father. Dougal may not know that Kieran isn't his biological daughter, but no one's cruel enough to ask him. Kieran didn't know for sure until YHD. Note that this has no bearing on Kieran being his heir or daughter - in any legal or social sense, she is his rightful daughter.
Kieran and Wolfram knew each other, and Wolfram was aware she was his sister, but he didn't say anything to anybody. Most others knew only that Manfred had a daughter - not who she was. Aldrich not only knew, but arranged to be Kieran's marriage broker when the time came. (That time being YHD.)
Kieran is sole heir to the domain of Donaghie - sole surviving aristocrat, in fact - but cannot rule in her own right. And Donaghie is bankrupt. She became betrothed to Sylvain von Tarkenburg during YHD. She's still too young to marry, but... she would like to give her father a grandchild so that he can finally die in peace - he's been so desperately ill for so very long.
When last seen, Kieran was headed back to meet The Tark and then back to Donaghie to help Sylvain take over. Then she was considering a move to the Bielenfeld Institute to get her father better medical care and herself some real education.