Stephen was the son of «u»Eudokia Ingerina «/u» and, officially, Emperor «u»Basil I «/u». However, at the time when he was conceived, Eudokia was the mistress of Emperor «u»Michael III «/u». Consequently, it is possible or even probable that like his older brother «u»Leo VI the Wise <http://en.wikipedia.org/wiki/Michael_III>«/u», Stephen was actually Michael's son.
Castrated by Basil I, Stephen became a monk and was designated for a career in the church since his childhood.