Leo 63 was a priest at Constantinople, in 785 he was sent as apokrisiarios of the newly elected patriarch Tarasios 1 with a letter from the emperors (Konstantinos 8 and Eirene 1) to pope Hadrian I (Hadrianos 1) at Rome containing a statement of the emperors' faith (and also Tarasios 1's synodical letter, see below); he travelled via Sicily where the strategos (Theodoros 13) sent him on to Rome in company with the bishop of Katana, Theodoros 70; at the Council Theodoros 70 mentioned these events during the second session, referring to Leo 63 as τὸν σὺν ἐμοι δοῦλον τῆς ὑμετέρας ἁγιωσύνης Λέοντα τὸν θεοφιλέστατον πρεσβύτερον; Leo 63 had earlier been referred to, but not named, as the apokrisiarios of the patriarch (τῷ εὐλαβεστάτῳ ἀποκρισιαρίῳ τοῦ ἁγιωτάτου ἡμῶν πατριάρχου): Mansi XII 1075-1078. He also delivered to Hadrianos 1 the synodikon of Tarasios 1 (διὰ Λέοντος τοῦ εὐλαβεστάτου πρεσβυτέρου ἡμῶν): Mansi XII 1077 (reply of Hadrianos 1 to Tarasios 1). As the representative of the patriarch, he presumably remained in Rome when Theodoros 70, with papal representatives and others, left for Constantinople in late 785 (see the letter of Hadrianos 1 at Mansi XII 1076).

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