Significant-Positive: Administering the virus / Hiyoko's death.

Details: Won in Nafuna's game.

So this memory starts with Ryouta coming to the infirmary after school because of his weak stomach. Shuu drugs him and activates the Charon virus in his body. Not too long after, his friend, the human student Tosaka Hiyoko, comes to visit. Here's the entire incident from Ryouta's POV:

Everything is hazy.
I think my stomach is acting up again.
I drink the medicine Doctor Iwamine gives me and lie down, but my body still feels heavy. I'm tired.

"Are you okay, Ryouta?"
"Yeah... I'm fine now."
"Mister Nanaki said you were coming here, so I wanted to make sure you were alright..."

Hiyoko is reaching towards me.
"Want to go home? I'll walk you."

Her hand stops.

Hiyoko falls to her knees, clutching her chest.
"...Ryouta... I can't breathe..."

Oh.
She can't breathe.

This is bad...
My mother said to get home early.

Everything feels strange.
Almost like it's a dream.

...I need to go home.

I also wrote a ficlet on this scene for my app to Kyriakos, which I'm going to report because I liked it.

It was quiet in the infirmary, which was how he preferred it. The medicines in their rows, the illegal, experimental medicines in their own rows, and no patients to annoy him or mess up his neat, pristine office.

The only sound of significance was Ryouta sleeping. Or rather, Ryouta lying unconscious and drugged as Shuu waited for his unfortunate human friend to arrive in search of him. He had gulped down the sedative without complaint, believing that it had merely been a new medicine Shuu had devised for his stomach. It was true, in a manner of speaking; the concoction was just the first step in Shuu's plan, which would ultimately result in Ryouta's stomach never bothering him again.

Ryouta tossed and turned in his sleep, wracked by uneasy dreams. Despite himself, Shuu found his thoughts drifting to Ryuuji's last days; his dull feathers, the persistent cough. Unlike his son, though, Ryuuji's final sleep in a hospital bed had been marked by the terrifying stillness of someone whose body was too weak to carry on.

Shuu shook off his thoughts. Now was not the time for maudlin reminisces, not with Operation Hatoful's completion so close at hand. Much depended on this afternoon. He busied himself with checking Ryouta's vital signs and then the time. School would almost be out for the day. Soon everything would be complete. Soon his obligation to Ryuuji would be fulfilled.

There was a knock on the infirmary door, the distinctly sharp, impatient knock and muffled shout that could only belong to a certain human girl, a specific Miss Tosaka. Shuu brushed a wing over Ryouta's forehead and allowed himself a private smile of anticipation as Miss Tosaka opened the door.

So yeah Shuu deliberately drugged the hell out of Ryouta and let him watch, barely conscious, as his best friend died in front of him.

Results:
- Recognizing Hiyoko's symptoms as the Charon virus!
- He knew this was all going to happen?? What did he do to Ryouta exactly.
- Ficlet aside Shuu won't have confirmation that Ryouta is Ryuuji's son but you know drugged!Ryouta lying in a hospital bed looks an awful lot like dying!Ryuuji lying in a hospital bed... So he'll have a sinking feeling that that's what the connection is. In which case, WTF AM I DOING TO RYUUJI'S SON???
- Anyway many feelings of excitement/anticipation/triumph and so on. IT'S ALL GOING TO ACCORDING TO PLAN AND IT'S ALL ABOUT TO BEGIN. 8>