The third questioner was a relative of Malchus whose ear Peter had cut off in Gethsemane ( John 18:10). Only John recorded the relationship. This fact supports the view that the "other disciple" was John. He knew the relationships of people within the high priest"s household. [source][source][source]
The accuser also identified Peter as a Galilean ( Matthew 26:73; Mark 14:70; Luke 22:59). His question expected a positive answer in contrast to the former two that expected a negative answer. It posed the greatest threat to Peter"s security. Peter responded by uttering his most vehement denial. Immediately a cock crowed (for the second time, Mark 14:72) fulfilling the prediction that Jesus had spoken just hours earlier (13:38). John also omitted Peter"s oaths and curses (cf. Matthew 26:74; Mark 14:71), Jesus" convicting look ( Luke 22:61), and Peter"s bitter tears of contrition (cf. Matthew 26:75; Mark 14:72; Luke 22:62). The effect is that the fulfillment of Jesus" prediction receives the emphasis. [source][source][source]
The encouraging record of Peter"s restoration to fellowship and usefulness follows in chapter21. [source][source][source]