According to Acts 1:15-26, Matthias was chosen to take Judas Iscariot's place among the apostles:
So one of the men who have accompanied us during all the time that the Lord Jesus went in and out among us, beginning from the baptism of John until the day when he was taken up from us—one of these men must become with us a witness to his resurrection.” And they put forward two, Joseph called Barsabbas, who was also called Justus, and Matthias. And they prayed and said, “You, Lord, who know the hearts of all, show which one of these two you have chosen to take the place in this ministry and apostleship from which Judas turned aside to go to his own place.” And they cast lots for them, and the lot fell on Matthias, and he was numbered with the eleven apostles (Acts 1:21-26-ESV)It's not as though there is no Rufus in the New Testament, however. According St. Mark's account of our Lord's Passion, Simon of Cyrene, who was enlisted to help Jesus carry His Cross, was the father of Alexander and Rufus, indicating that Simon and his sons were members of the early Christian community, perhaps known to some of the readers, or at least known to the author of the Gospel. In this regard, it is interesting to note that a very ancient tradition holds that the origin of Mark's Gospel is the preaching of St. Peter in Rome.