“Underlying reality” is a way of understanding hypo-stasis (literally, what is under), but it’s rather the acception of medicine, as far as I’m aware of. Christian theology, I think, interprets it differently, as in sub-ject (the same word, but based on Latin rather than Greek roots) – that is, “person”.The word person did not exist as how you’re using it it was hypostasis which literally means an underlying reality or existence subsistence so to say the word that I’m looking for is person is to deny that the word that is used is hypostasis.
(By the way, the word “subject” has the same plurality of meanings : it can be as well “subject matter” as “individual”).