From BiblePortal Wikipedia

Hastings' Dictionary of the New Testament [1]

(Νηρεύς, a Greek name, fairly common among slaves and freedmen, and found in inscriptions of the Imperial household)

Nereus is the third of a group of Christians, his sister (probably Nereis or Nerias by name) being the fourth, who with ‘all the saints that are with them’ are saluted by St. Paul in  Romans 16:15. The first two names, Philologus and Julia (which see) may be those of husband and wife. If so, Nereus and his sister and Olympas may have been their family, which formed the nucleus of a church which met under their leadership at their house in Rome or Ephesus. Cf. possible the ‘household of Stephanas’ in Corinth, who were ‘the firstfruits of Achaia’ and who ‘set themselves to minister unto the saints’ ( 1 Corinthians 16:15). The relationship is, however, purely conjectural, as nothing further is known of any of these persons. That they formed with the other unnamed persons a household or district (ἐκκλησία), of which they had been the nucleus and therefore became the leaders, is extremely probable, or the men may have been the heads of separate small communities. The name Nereus was that of a minor sea-god, father of the Nereids, and it is significant that a Christian should have had no scruple in retaining it. (Other names of heathen deities borne by Christians mentioned in Romans 16 are Hermes [ Romans 16:14], Phoebe, [ Romans 16:1].) The name is connected with legends of the early Roman Church (see Sanday-Headlam, International Critical Commentary, ‘Romans 5,’ Edinburgh, 1902, p. 428).

T. B. Allworthy.

Hastings' Dictionary of the Bible [2]

NEREUS. A Roman Christian, to whom, along with his sister, St. Paul sends greeting in   Romans 16:15 . The expression ‘and all the saints that are with them’ seems to point to some community of Christians accustomed to meet together.

Morley Stevenson.

Fausset's Bible Dictionary [3]

A Christian at Rome whom Paul salutes ( Romans 16:15). Of Philologus' and Julia's household, Origen guesses. Tradition makes him to have been beheaded at Terracina under Nero, and his ashes deposited in the church of Nereo and Archilleo at Rome.

Smith's Bible Dictionary [4]

Ne'reus. (Lamp). A Christian at Rome, saluted by St. Paul.  Romans 16:15. According to tradition, he was beheaded at Terracina, probably, in the reign of Nerva.

Hawker's Poor Man's Concordance And Dictionary [5]

A friend of Paul's, ( Romans 16:15) derived from Ner.

Morrish Bible Dictionary [6]

Christian at Rome to whom Paul sent a salutation.  Romans 16:15 .

Holman Bible Dictionary [7]

 Romans 16:15

Easton's Bible Dictionary [8]

 Romans 16:15

International Standard Bible Encyclopedia [9]

nē´rūs , nē´rḗ - us ( Νηρεύς , Nēreús ): The name of a Roman Christian to whom with his sister Paul sent greetings (  Romans 16:15 ). Nereus and the others saluted with him ( Romans 16:15 ) formed small community or "house church." The name of the sister is not given, but the name Nereis is found on an inscription of this date containing names of the emperor's servants (Lightfoot, Phil , 176). Among the Acta Sanctorum connected with the early church in Rome are the "Acts of Nereus and Achilleus" which call them chamberlains of Domitilla, the niece of Vespasian, and relate their influence over her in persuading her to remain a virgin.

Cyclopedia of Biblical, Theological and Ecclesiastical Literature [10]

(Gr. Νηρεύς ), a marine divinity in classic mythology, was represented as a wise and prophetic old man, and was believed to dwell at the bottom of the sea with his beautiful daughters the Nereids. He was regarded as ruling principally over the iEgean Sea, and was believed occasionally to appear to men in different shapes, predicting what should befall them in the future. The poets feigned that he could assume various forms like Proteus, and would only reveal the future when, having exhausted his powers of transformation, he was reduced to his original shape. Nereus yielded his place to Poseidon, and gave him his daughter Amphitrite. His attribute was the trident. He frequently appears in ancient works of art.

The Nuttall Encyclopedia [11]

The god of the Mediterranean Sea, the son of Pontus and Gaia, the husband of Doris, and father of the Nereides, represented as a sage, venerable old man.