Zenitsu is young when he realizes he's different than other people.
He doesn't know exactly what made him understand that, but later, when he'll think about it, he will wonder how the hell did he not notice something was amiss with him. How he didn't notice how sometimes, when he talked to the nice old lady who was always seating by the river or the little boy close to the woods where bears hide, the strange looks people gave him, the way they would watch him with someting between incredulity and fear. How he didn't remember (didn't want to) that time when he asked a young man why he was ignoring the beautiful woman besides him, and how said woman had smiled so sadly while the man sobbed.
But if he had to pinpoint the exact moment the thought It isn't normal crossed his mind, it would be one of his fights with Kaigaku.
It's not a special fight or anything. It's just another time where his senior mocks and insults him while their professor isn't here. It's a daily occurrence, frankly.
They're eating, and Zenitsu is trying his best to ignore the constant and hurtful jibes when Kaigaku utters something that has the effect of an explosion.
“I bet you would kneel in front of any demon, begging for mercy like the pathetic brat you are ! I would never step as low as you !”
The scream makes him jump, and he frantically looks around him. Kaigaku stares at him like he's some sort of disgusting insect (like always) but otherwise doesn't react. Zenitsu doesn't understand how he can stay so calm after hearing the rage and hatred in this scream.
“LIAR ! MURDERER ! TRAITOR ! HYPOCRITE !”
And the screams don't stop. It's like a bunch of kids are yelling all their despair and hatred towards someone and Kaigaku isn't reacting. Zenitsu knows his hearing is superior to the norm but everyone should be able to hear the agonizing screams of these children.
Then he looks back at Kaigaku and his heart stop.
At first he thinks it's a demon. But a closer inspection makes him see the silhouettes of children, mangled together in a dark and bloody mess. Some are almost transparent, but others have wounds so clear it makes him want to puke. They are superimposed, and they're so close to Kaigaku that some of them can touch him.
And they try to. A bloody hand attemps to grab Kaigaku's throat only for it to pass through, and a little mouth tries to bit him in the shoulder without any success. And Kaigaku doesn't blink or flinch or react at all. He just stares at Zenitsu like he's the weird one.
And Zenitsu faints.
No one believes him, of course. Who would ?
So he learns to live with it. It doesn't take him long to figure out the monsters are actually ghosts.
They are different from each other. Some look so human that Zenitsu has to check and see if they have a heartbeat to be sure. Others are so mutilated and full of hatred that they can only scream incoherents things. Those he avoids at all costs.
Sometimes, he can briefly see a shadow or hear a whisper of someone who stopped existing a long time ago. Strangely, those ones are almost comforting.
He doesn't talk to them. They're not a lot to begin with, and sometimes he can only hear and not see. The ghosts haunting Kaigaku are strange, sometimes so loud that Zenitsu's ears bleed and sometimes so quiet that if he hadn't seen them, he would think they weren't there.
The few he talks to are former demons slayers. They're three in the temple, two children, a boy and a girl around fifteen years old, and a man named Nozomu. This one has a strange mark on his left eye, golden and like lightning, and he looks so tired that Zenitsu can't help but worry about him. But when he asks him if he's okay, after making sure they're alone, he just smiles sadly and says :
“You're a kind one.”
It's the first and last time Zenitsu hears his voice. He's not here the next day, or the one after.
When he asks the two other ghosts about it, they exchange a glance and the girl simply says :
“I guess he found what he was looking for.”
They never talk about Nozomu again.
But seeing ghosts doesn't help Zenitsu with his training. They try to help, but the girl, Saikyo Aokawa, is a user of Breath of Water and the boy, Kireina Senshi, of Breath of Flower. They're strong, but died young and don't know a lot about the Breath of Thunder.
(Zenitsu wonders why they're here, in a temple where the thunder is taught, and if they died here, and how long ago. But he doesn't ask and they don't tell.)
They still help though, but in a different way. They whisper encouragements and compliments after training, and insult Kaigaku when he tells him how pathetic he is and will always be. Aokawa is rather creative when it comes to insults and threats, but Senshi is even more frightening. Kaigaku's things suddenly start to disappear or abruptly fall on his head. They laugh together for hours later, lying on the grass at night when no one will wonder why he is laughing alone like that.
It doesn't last, of course. Aokawa smiles at him one day and kisses his forehead before dissolving like water, an invisible weight gone from her shoulders.
Zenitsu cries, but Senshi ruffles his hair and says brightly :
“Don't worry about her. She's with her family now.”
“But why now ?” Zenitsu whines, because Aokawa was one of his only friends and she just left without as much as a goodbye.
Senshi doesn't answer, just hums and hugs Zenitsu.
He trains harder. It's weird without the yells and cheers of Aokawa, but the kind words of Senshi keep him going.
The Final Selection happens. He's alone this time, without either of his friends, but it barely registres because there are so many ghosts it's not even funny. Most of them are visible and audible, and wearing a strange mask on the side of their heads. They also all disappear during the first day, which is even weirder, but not the weirdest he has seen. That should probably worry him.
He passes. He comes back bruised and bloody but alive and with a sparrow in tow and he's just so happy because look Gramps, Senshi, your encouragements helped me !
Said ghost beams when he sees him, and Gramps cries, and they have a dinner to celebrate. It's great, even if Zenitsu is scared about the future.
He falls asleep with Senshi by his side, and wakes up alone.
They are all gone now, he thinks, bitter and sad and wiping his tears away, and not even Gramps's pride is enough to cheer him up. His last friend just left, before Zenitsu could show him how much he was grateful and it hurts.
(He can't mourn, because they were already dead, the three of them, and that's what hurt the most.)
After that, he swears to never talk to ghosts again, because the living shouldn't frequent the dead unless they want to be hurt. It goes on for weeks.
And then he meets a boy with a weird box on his back and an even weirder and expressionless ghost, and his plans are thrown out of the window.