Hi Bilcu1;
That’s a very interesting question. I’m very interested myself in the notion of person, which has quite a rich philosophical and theological history.
Aquinas says that a person is “an individual substance of a rational nature.” That requires a little unpacking! In fact, Aquinas has quite a developed philosophy on what it means to be a person.
He starts with the notion of Person as it applies to God: “Person” refers to a relation which subsists in the divine nature. In other words, the nature of God is that he is relational. Because humans are made by God (and made in His image) we are likewise relational. So the term person essentially refers to a being-in-relation.
We can’t understand there being three persons in One God, because we are one person in one substance. But we have to be careful about trying to figure God out but referring him to what we are. Rather, what we should be doing is trying to figure out what we really are, but learning about what God is, as best we are able. What we know of God, since He has revealed this to us, is that personhood is all about relation. And our personhood is incomplete without God, who invites us to ‘complete ourselves’ so to speak, in relationship with Him.