Killua's POV

I walked into school for my second day. I got there a little late, so when I walked into the classroom, everyone was staring at me. 'Damn, what did I do?' I thought to myself while moving toward my desk in silence.

Eventually, class got in session and we went through a normal, boring day of school. We went to lunch and I sat at a table all by myself. For obvious reasons, everyone was too intimidated to talk to me, so I sat alone. We got back to the class and then the one guy, I think his name was All Might, came through the door saying, "I am coming through the door like a normal person!" while totally not coming in normal, at all.

Everyone around me was geeking out, but I wasn't amused. I didn't care about this guy, so why should I act like I do? Anyway, he then announced that we were doing battle training and that we got our costumes. Mine was just my normal outfit, a blue turtleneck with a white t-shirt over it, blue shorts, and purplish-red shoes. 

Third Person POV

Some of the students who happened to be faster at changing, Killua, Iida, Bakugou, and Kirishima, walked out before the rest of the students. They all noticed one thing right away, Killua's outfit was very simple. Even All Might was set back by this. He looked at Killua and said, "Young Zoldyck, are you sure you want your outfit to be so... simple?"

Killua frowned and the others stared at him. 'So his last name is Zoldyck?' they all thought.

"First of all, don't use my last name, just call me Killua. Second of all, my outfit is just fine, thank you very much." Killua said sassily.

"But it's custom here to call people by their last name," All Might said, confused.

"Does it look like I care? I'd rather not be reminded of my family every time someone speaks to me." Killua said quietly, but loud enough for all of them to hear.

'Does he have a bad family?' they all thought.

Before anyone else could say anything, the rest of the class came outside. 

Killua's POV

"You are going to be split into teams of two by random." said All Might.

The crazy robot man, Iida, right? raised his hand and said, "There are 21 of us sir, how do you expect an even split?"

"Ah yes, Young Zol- I mean, Killua, will join up with one of your groups," he replied.

I was put into Uraraka and Midoriya's group. It was okay, I guess. We got inside and were ambushed right away. I bolted at Bakugou and sent a shock of electricity at him. I got him down temporarily, and looked away for a moment to see Uraraka go. When I turned back, Bakugou was fighting Midoriya. I couldn't see a way to get into the fight without hurting Midoriya, so I stayed back. I saw Midoriya run away and I took my chance to strike. I lunged at Bakugou and hit him hard in the face. He was knocked down for a moment before getting up. We sparred, but he couldn't hit me. I, on the other hand, hit him many times. Hard. Eventually, I pushed him back and he looked at me, enraged. He raised his gauntlet at me and said, "You know, this thing stores my nitroglycerin sweat. The more sweat I have, the bigger the explosion I can make. This thing is packed full!" before pulling the pin.

I watched the light of the explosion grow in his hand, until I was overwhelmed with heat and light. I was brought back to the worst time of my life.

Flashback

"Jan"

The sphere around Gon's stump of an arm grew.

"Ken"

It grew to be larger than him, and it started to swallow him whole.

"GON!" I screamed at the top of my lungs, watching as my best friend exploded and was burnt beyond recognition.

End of flashback

I didn't even feel the pain of the burns around my body, all I saw was Gon, looking back at me, crying. All I saw was him lying on a hospital bed, on the edge of death, burnt to a crisp. So far beyond recognition that it broke my heart.

My face paled, I fell to the ground and felt the tears flow down my face.

"I'm sorry... Gon..." I said.

I heard some more loud noises, and eventually they stopped, but I didn't care.

"Gon! I'm so sorry!" I cried out, sobbing into the floor.

I felt someone grab my shoulder, and I was snapped out of my misery. I looked up and saw Bakugou, Iida, Uraraka, and All Might staring at me. I wiped the tears from my eyes, stood up, and walked away. I was stopped by All Might, who said, "Young Killua, are you alright? What happened?"

"None of your damn business." I said.

I walked back into the building the rest of the class was in and saw them all staring at me. 'Great, now they won't ever stop asking questions.' I thought to myself.

I stood off in the corner and watched the rest of the class. As I was walking out, a few people came up to me and asked, "Hey Killua, who's Gon?"

My eyes widened with shock, "How do you know about him?"

"Well while you were freaking out, you said his name a bunch followed by a bunch of apologies." Kaminari said.

I sighed and said, "He's my best friend, now leave me alone."

"No, Killua. We want to help you. We're your classmates now and we have to help." Iida said.

"Why do you care so much, it's not like we're friends. I've already gotten over it so there's no need to get into my personal life!"

"That display earlier would say otherwise." Yaoyaorozu said.

I shouted, "I said I'm FINE!" before storming out of the room.

I was running, and eventually I ended up back at Aizawa's apartment. I ran in and slammed the door to my room. I looked in my pocket and I saw a picture of me, Gon, and Alluka. I hugged the picture and began to cry. Silently at first, but then sobs wracked through my body. After I calmed down, I looked at the picture again. I sniffed and said, "I miss you guys. I'll get back to you, I promise." 

I lay there for a while and I eventually fell asleep. 

Aizawa's POV

I got a call from All Might telling me what had happened. After taking an explosion head on, Killua dropped to his knees, began to cry, and started shouting apologies to a person named Gon. The events from the night before ran through my head. While still on call with All Might, I said, "I know what's happening. He must have PTSD. He had a dream last night where he was crying and was screaming the name Gon. I think he saw this person, Gon, get either killed or seriously injured in an explosion."

All Might was silent, I bet he was processing how a 15 year old boy could have serious PTSD. It was still a large pill to swallow myself, but the evidence was all there. I hung up and entered my apartment. I heard crying coming from Killua's room. My heart was crushed, but I silently approached his room. I put my ear to the door and heard him say, "I miss you guys, I'll get back to you, I promise."

His room went silent and after a few minutes, I walked in. I saw him asleep on his bed, clutching a photo. I slowly took the picture from his arms and looked at it. It had him but he looked a lot younger. He was actually smiling and looked genuinely happy. A young girl with black hair, a pretty kimono, and odd hair pieces that resembled faces stood next to him. A boy also stood next to him. The boy next to the girl had blackish-green hair and was wearing all green.

'The boy must be Gon, but who's the girl?' I thought to myself.

"Well, except my little sisters Alluka and Nanika..." the memory flashed through my head. 

'So that must be one of his sisters' I thought, 'He looks so happy in this photo. What could have happened to change him?'

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A/n: Hi, so Killua has PTSD. I really do think that anyone after seeing what Killua saw would get a form of PTSD. Also, I know that the photo is from after the whole event with Gon happened, but still. After the event, he could have been suppressing the emotions, but after years of maturing, and all the stress of finding himself in another dimension where he can't contact anyone he knows or loves, he would be pretty prone to a mental breakdown.  Anyway, thank you so much for reading!