Relation in the Trinity, St. Thomas
“Distinction in God arises only through the relation of origin. However, a relation is in God not as an accident inherit in a subject, but it is the divine essence itself; therefore it is subsistent just as the divine essence subsists. So, just as Godhead is God, the divine Fatherhood is God the Father, who is a divine person. Hence ‘divine person’ signifies a “relation as subsisting”. This means a relation in the manner of a substance which is a hypostasis subsisting in the divine nature; though what is subsisting in the divine nature is just the divine nature itself.”
St. Thomas Aquinas, S. Theol.
‘A relation in the manner of a substance which is also a hypostasis subsisting in the divine nature’ – this is St. Thomas’s mature and considered understanding of ‘person’ as applied to the Father, Son. and Spirit.