Which 'brother' is named with Paul as co-sender at the start of 1 Corinthians?

Answer: Sosthenes

The letter opens, "Paul, called to be an apostle of Christ Jesus by the will of God, and our brother Sosthenes."

“Paul, called to be an apostle of Christ Jesus by the will of God, and our brother Sosthenes,”— 1 Corinthians 1:1 (BSB)