At SB 3.8.7 Sanatkumara is called 'nivR^itti-dharmAbhiratAya', one who delights in the dharma of withdrawal, and the most excellent of the Lord's own told this to him. Being asked, he told it on to Sankhyayana, who had taken vows. He is one of the Kumaras, but this verse names him individually.
Sanatkumara
सनत्कुमार
Sanatkumara is the sage, one of the Kumaras, who in the lineage of the Bhagavata received the teaching from Sankarshana. At SB 3.8.7 Maitreya says: 'proktaM kilaitad bhagavat-tamena nivR^itti-dharmAbhiratAya tena / sanatkumArAya sa Aha pR^iShTaH sA~NkhyAyanAyA~Nga dhR^ita-vratAya' — the most excellent of the Lord's own told this to Sanatkumara, who delights in the dharma of withdrawal, and on being asked Sanatkumara told it on to Sankhyayana, who had taken vows. He belongs to the company of the Kumaras, but this verse names him individually, which is why he is held as an entity distinct from the group.
Śrīmad Bhāgavata Mahāpurāṇa 3.8.7 — Sanskrit Wikisource (pinned chapter page; Devanagari from the cross-validated transcription in validation/sanskrit_skandha3.json)
Sanatkumara is the student of Sankarshana. Source evidence (SB 3.8.7): At SB 3.8.7 it is said that the most excellent of the Lord's own told this to Sanatkumara, who delights in the dharma of withdrawal: 'proktaM kilaitad bhagavat-tamena nivR^itti-dharmAbhiratAya tena / sanatkumArAya sa Aha pR^iShTaH sA~NkhyAyanAyA~Nga dhR^ita-vratAya'. Additional source (SB 3.8.7): Sanatkumara is the student of Sankarshana. Source evidence (SB 3.8.7): At SB 3.8.7 Sanatkumara is described as having been told this by the most excellent of the Lord's own, and on being asked he told it on to Sankhyayana.
Son Brahma DeitySanatkumara is the son of Brahma. Source evidence (SB 3.12.4): At SB 3.12.4 Sanatkumara is counted among the four sages created by Atma-bhu, Brahma.