In Zechariah's vision, who stood at the right hand of Joshua the high priest to accuse him?

Answer: Satan

Joshua stood before the angel of the LORD with Satan at his right hand to accuse him (KJV: "to resist him"), and the LORD rebuked Satan.

“Then the angel showed me Joshua the high priest standing before the angel of the LORD, with Satan standing at his right hand to accuse him. And the LORD said to Satan: “The LORD rebukes you, Satan! Indeed, the LORD, who has chosen Jerusalem, rebukes you! Is not this man a firebrand snatched from the fire?””— Zechariah 3:1-2 (BSB)