Now that the identity of experimental subject 1 is known, I am sure that the author placed some hints in the first chapters of the manga.
One of them, I believe is
When Arata hurts his knee, she asks if that is how a teenager wound should heal.
What were most/all oof them?
I noticed a significant hint in episode 5 while rewatching in dub form at Funimation. I did not catch it during my first viewing of the sub at Crunchyroll.
Begin at about time mark 11:30. Kaizaki has been speaking with Hishiro concerning why her backpack had been taken. She doesn't understand why Rena would hate or be jealous of her. Kaizaki realizes how socially awkward Hishiro is, and comments that she wouldn't get it. She already knows about her awkwardness, and the bald statement hurts a little. This was indicated by the cartoon lightning the show occasionally uses for an unintentional word "attack". // Immediately Hishiro has a short flashback to someone else telling her the same, about her inability to "get it". What can be discerned in the dub, but was not obvious to me in first viewing the sub, is that an adult woman is speaking to her with disdain. Also I had assumed that the person shown (from their back) was the speaker. With 20:20 hindsight, it is clear that instead this was Hishiro herself as an adult.
I have noticed other minor hints, but I don't remember what they were or what episode. This was the only one, though, that made me think, "Geez, I missed that?!?".
Adding another hint (4 days after first answering). As I started watching episode 9, pre-opening credits:
Honoka and Hishiro talk about going to lunch at the end of class. Chizuru wonders about Rena, and Honoka states she isn't here today. The two go off to lunch together. The whole time Ryo was sitting next to Hishiro, and as they leave it is clear he was intently observing the two. I am guessing he is pleased to see Hishiro showing concern for a friend. // Another occurs near the middle of episode. An and Ryo are talking in their office, and An casually says "our subjects". The first time viewer will think generally, as in the subjects of ReLife Corporation. But now it is clear she meant Kaizaki and Hishro specifically.
