Achaicus in the Bible Encyclopedia - ISBE

a-ka'-i-kus (Achaikos, "belonging to Achaia"): A name honorably conferred upon L. Mummius, conqueror of Corinth and Achaia (compare CORINTH). Achaicus was one of the leaders of the Corinthian church (to be inferred from 1 Cor 16:15 ff) who, visiting Paul at Ephesus with Stephanas and Fortunatus, greatly relieved the Apostle's anxiety for the Corinthian church (compare 1 Cor 5:1 ff). Paul admonishes the members of the Cor church to submit to their authority (compare 1 Thess 5:12) and to acknowledge their work (1 Cor 16:15 ff).

Link: https://bible-history.com/isbe/A/ACHAICUS/