The most obvious is Felix's memory issues. His brain has difficulties retaining information and he'll often forget minor things such as a date, his wallet, or the current time.

The antique shop is owned by Hyunjin's grandmother. He used to go there all of the time with Hyunjin to visit the old woman and drink tea with her, in addition to helping her with the shop. The entire place was basically wiped from his mind after his last mental crash and, for a while, he simply walked past it without ever noticing it, unconsciously rejecting the very existence of the shop from his mind.

It's not the first time Felix wanders into the shop after losing his memories, hence why the old woman barely reacted. She's grieving her grandson's death, but also the loss of Felix, whom she cares a great deal about – but she's given up trying to reason him, hence the short interactions.

The mirror with which Felix cut himself used to belong to the couple. It was in their room, and Hyunjin would spend hours in front of it, picking at his "flaws" and judging himself over and over. It's also the mirror he eventually shattered in a burst of anger, and of which he used one of the shards to kill himself. It was eventually repaired and put up for sale in the antique shop because Felix couldn't possibly keep it.

Minho accepted that Felix forgot about them being brothers since it meant less pain for Felix, but he's still very protective of the younger, of course – hence why he acted as if the antique shop and Hyunjin didn't exist. He's also the one who urged the other boys not to say anything about it to Felix and act as if Hyunjin simply never existed.

Felix has actually been friends with Jisung for years, but he also forgot about him because of how closely he was tied to Hyunjin (they met through the older). Felix actually forgets his wallet every Thursday morning because Hyunjin used to buy coffee for the both of them every Thursday morning.

Ballet was Hyunjin's favorite, and Felix met him in that class. After they started to date, they began sharing a dream of starting their own dance school, and they kept up with ballet classes as a hobby.

Minho started using to deal with Hyunjin's death, and then to deal with his little brother's descent into madness.

All of the dreams Felix has of Hyunjin dancing is actually a metaphorical expression of the guilt that's eating him away – his feelings of guilt, powerlessness, and failure that are driving him insane along with his grief.

Seungmin didn't have a nightmare. He woke up and Felix wasn't there and all he could think about was how he'd found Felix cutting the last time Felix locked himself in the bathroom. He's terrified of losing Felix like he lost Hyunjin.

Felix wasn't pushed in the street. As the bystanders saw, he practically threw himself in front of the car. All of the "attempts" on his life were basically him resisting the unconscious urge to kill himself.

Chan and Changbin are old friends who took Kkami after Hyunjin passed away. Felix was in no state to take care of the dog so they volunteered to take care of it until Felix recovered – but, as it happened, Felix never did recover, and ended up forgetting about Kkami altogether, so they kept the dog.

Hyunjin kept on telling him not to sign the lease for the gym, but those were actually intrusive thoughts, because he knew deep down that he wasn't going to live for much longer.

The cut that kept on reappearing on his palm was simply him cutting it over and over again, hence why it never healed.

When Felix is ranting to his friends, they keep up the charade of not knowing who he's talking about because they're scared of how badly Felix will react should he remember Hyunjin properly. Also, he's not being strangled – it's just a panic attack.

While Minho did start to take drugs after Hyunjin passed away, the relapse their mother mentioned is Felix's.

The music box is a gift Hyunjin received from his grandmother when he was just a child, and the little melody was his favorite. It actually broke a few months before Hyunjin killed himself, and it only added to the weight already destroying him from the inside. As such, any music Felix thought came from the music box... well, didn't.

"The couple walking their dog" are Chan and Changbin. They walk Kkami in the park every week and Felix often crosses path with them, but he usually forgets all about it the moment they part ways.

Chan and Changbin are also the ones who composed the song to which Felix danced in his studio. They made it using the melody from Hyunjin's music box, and Felix and Hyunjin then created their own choreography to go with it – it was their song, hence why Felix knew all of the steps.

Felix didn't cut himself while breaking the mirrors in the gym, but when breaking the antique shop's window to steal the music box.

As told by the police offer, Felix died the same way Hyunjin did – by slitting his own wrists using a mirror shard.

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All in all, this story is double-layered, and told from an unreliable narrator's point of view. You can either choose the supernatural explanation, or you can choose the psychological one – both work, and both have some truth to them.