He was speaking about Judas, the son of Simon Iscariot. For although Judas was one of the Twelve, he was later to betray Jesus.
Textus Receptus (TR) - John 6:71 >Ἔλεγε δὲ τὸν Ἰούδαν Σίμωνος Ἰσκαριώτην· οὗτος γὰρ ἤμελλεν αὐτὸν παραδιδόναι, εἷς ὢν ἐκ τῶν δώδεκα.