Which young disciple, son of a Jewish believer and a Greek father, joined Paul in Acts 16?

Answer: Timothy

At Lystra Paul found Timothy, son of a believing Jewish woman and a Greek father, and took him along on his journey.

“Paul came to Derbe and then to Lystra, where he found a disciple named Timothy, the son of a believing Jewish woman and a Greek father.”— Acts 16:1 (BSB)