ISB ~ John 18 v15

another disciple:Probably John the evangelist, who never reveals his name in the Gospel but often calls himself the disciple “whom Jesus loved” (13:23; 19:26; 20:2; 21:7). One tradition preserved by Eusebius holds that the Apostle John was born of a Jewish priestly family, which could explain his familiarity with the high priest (18:15), the name of the high priest’s slave (18:10), and the family of the slave (18:26).

See introduction to John:Author.