I have 3 more theories to add for this drama – I hope I didn’t go overboard.

1st theory (which is just an elaboration of conjecture #1)

This is revisiting conjecture #1: I keep seeing hints of it –
(1) Ri Ta says that he was warm in her memory, when he placed medicine on her forehead, as warm as he is now with human temperature. This is a big hint to me, that being kind/feeling something close to love is what makes him human.
(2) He is calm whenever he colours pictures.In particular, they mentioned Jeju Islands and the woods, which always made him think of her. Yet another obvious evidence!

#3 Antidote for Ji Sung

Wasn’t the mother creating something when the infected came to kill her? I suspect that ultimately, the doctors are going to get a hint from the chemicals on the table and find out what the substance was that the mother hid in the drawer. Maybe it can help Ji Sung become human!

#4 Ri Ta’s uncle killed Ri Ta’s parents

Uncle actually was the one who killed Ri-Ta’s parents – he’s the mastermind and the Assistant Director is the one who probably hired the people to kill them. Remember those suspicious-looking park wardens?

This is a really obvious theory – all evidence is pointing towards that right now!