The plural 'let us make' reveals a co-creator
Eustathius is answering the philosopher's question about whom God addressed in Genesis 1:26, arguing that the plural verb introduces a co-creating person.
For by saying, “And God said, ‘Let us make a human being in our image and in our likeness’” (Genesis 1:26), the text throws in a question for us to understand: to whom was he saying “let us make”? After saying, “And God said, ‘Let us make a human being,’” it goes on to say, “And God made the human being; in God's image he made him; male and female he made them” (Genesis 1:27). So the meaning of “let us make” indicates that there is a co-creator, and it introduces one of equal address. For just as the Father who said “let us make a human being” is God, so also the one to whom he said “let us make” is God, since the deity of both persons is one—both the one who said, “Let us make a human being in God's image and in God's likeness,” and the one who made the human being. The voice “let us make” presents the all-holy and unvarying deity of the Father and the Son. For the image of God is simple and uncompounded, being fire in nature.
Greek
ἐν τῷ γὰρ εἰπεῖν· καὶ εἶπεν ὁ Θεός· ποιήσωμεν ἄνθρωπον κατ' εἰκόνα ἡμετέραν καὶ καθ' ὁμοίωσιν, ζήτημα ἐνέβαλε νοῆσαι ἡμᾶς, πρὸς τίνα ἔλεγε τὸ ποιήσωμεν· εἰπὼν γὰρ· καὶ εἶπεν ὁ Θεός· ποιήσωμεν ἄνθρωπον, ἐπήγαγε λέγων· καὶ ἐποίησεν ὁ Θεὸς τὸν ἄνθρωπον, κατ' εἰκόνα Θεοῦ ἐποίησεν
αὐτὸν, ἄρρεν καὶ θῆλυ ἐποίησε αὐτούς. Ἡ οὖν τοῦ ποιήσωμεν σημασία τὸ εἶναι πρόσωπον συνδημιουργὸν σημαίνει καὶ συνισοπροσήγορον εἰσάγει. Ὡς γὰρ Θεὸς ὁ Πατήρ ἐστιν ὁ εἰπών· ποιήσωμεν ἄνθρωπον, οὕτω Θεός ἐστι καὶ πρὸς ὃν τὸ ποιήσωμεν εἴρηκε, μιᾶς οὔσης τῆς τῶν ἀμφοτέρων προσώπων θεότητος, τοῦ τε εἰπόντος· ποιήσωμεν ἄνθρωπον κατ' εἰκόνα ἡμετέραν καὶ καθ' ὁμοίωσιν, καὶ τοῦ ποιήσαντος τὸν ἄνθρωπον. Τὸ γὰρ ποιήσωμεν τὸ πανάγιον καὶ ἀπαράλλακτον τῆς τοῦ Πατρὸς καὶ Υἱοῦ θεότητος παρίστησιν ἡ φωνή· ἡ γὰρ τοῦ Θεοῦ εἰκὼν ἁπλῆ καὶ ἀσύνθετός ἐστι, πῦρ οὖσα τῇ φύσει.