Who was Jesus' faithful witness killed in Pergamum, "where Satan dwells"?

Answer: Antipas

Pergamum held fast to Christ's name even in the days of "My faithful witness Antipas, who was killed among you."

“I know where you live, where the throne of Satan sits, yet you hold fast to My name. You did not deny your faith in Me, even in the days of My faithful witness Antipas, who was killed among you where Satan dwells.”— Revelation 2:13 (BSB)