* The Apostles: Stephen was one of the seven chosen men to help the apostles in their ministry (Acts 6:5). It's likely he learned from them and received guidance from them.
* Gamaliel: Gamaliel was a renowned Jewish teacher and Pharisee. He was the teacher of the Apostle Paul (Acts 22:3). While there's no direct evidence that Stephen studied under Gamaliel, it's plausible given the time period and the fact that Gamaliel was a respected figure in Jerusalem.
* Other Disciples: The early church was a close-knit community. It's possible that Stephen learned from other disciples who had been following Jesus for longer.
Ultimately, we can't definitively say who Stephen's mentor was, but it's likely he was influenced by several individuals within the early Christian community.