Erastus (v.20) — the chamberlain (treasurer) of the city of Corinth (Romans 16:23). He may also have been with Timothy in Macedonia (Acts 19:22)
Trophimus (v.20) — the innocent cause of Paul’s arrest in Jerusalem (Acts 21:29)
Verse 20 also emphasizes the fact that the sign gifts had indeed vanished. He had had to leave Trophimus at Miletum sick. If the gift of healing had still been in order what could possibly have been the reason for Paul’s writing to Timothy that he had left Trophimus, a faithful companion, sick along the way to Rome? — Stam, page 228
you (v.22) — plural — Timothy and those with him in Ephesus