I used the 'Person A is dead and Person B sees the dead' prompt from to start this. I hope you like it and maybe want more because I really enjoyed writing it.

John was always a little on the reckless side, when he was young he would climb trees and run across roads and it only really got worse as he grew up. Throughout high school he was known as that one kid who would do anything for a fiver, he once went to school for a whole month wearing pyjamas because he'd been dared to. So it wasn't particularly surprising when he was found at 2am with his car wrapped around a tree, wearing just his boxers a few miles away from his university campus.
Karkat however was always on the more careful side, when he was young he kept away from other loud children and held his mother's hand. Throughout high school he would act like the leader of the group, planning all group outing to make sure nothing went wrong, snapping at the more reckless people in his friendship group. When he met John he swore one day he was going to kill himself on a stupid dare and Karkat would laugh at him for being so stupid.
At John's funeral there was no laughter, the church was full of all the people who had known John, all equally distraught. At the moment John realised how stupid he'd been, how much he regretted every stupid choice he had made, and how confused he was now. It goes without saying John had died shortly after the initial impact of the car on the tree, but he was still here or at least it felt like that to him.
"Maybe this is my fate" he mused, "To watch as everyone suffers because of my mistake."
So he did, he watched, he watched as the priest stood in front of the rows of people, he watched as his father struggled through his tribe, he watched as his casket was carried to his grave and watched as his friends cried.
He watched for hours, days, weeks. He wandered between his home with his father, watching as he slowly cleared his room and between the rooms of his friends. They tried to say positive and not think about John but he caught them in the moments they did and he wished so much he could talk to them. Until one day, whilst watching Karkat he stopped watching, he sat next to his friend's slumped form, shaking slightly as he cried quietly.
"I'm sorry." He whispered.
Karkat looked up slowly frowning "What?" he said.
John jumped up from where he was in disbelief, there is no way Karkat could have heard that! But sure enough Karkat's eyes widened as they locked onto the pale form in from of him.
"John?" he whispered almost too quiet for John to hear, the colour fading from his face.
" what?" was all John could stutter out whist staring into Karkat's blood-shot eyes.
"Oh God…" Karkat muttered to himself whilst rising slowly. "Oh God!" he got louder starting to pace around the room. "I've gone crazy! Oh fucking God! This is it, holy shit, I've lost the fucking plot!"
John quickly moved towards Karkat trying to sooth him "Karkat no, it's me! It's really me!" he tried to place a hand on the boy but it went straight through him.
"NO!" Karkat suddenly jumped away from John. "You're not John! John's dead! He's dead and he's not fucking coming back and you're not him!" He shouted loudly slowly dissolving into violent sobs.
"I'm sorry" John finally said after a long pause, Karkat's loud crying being the only noises that filled the room. "I'm so sorry. I don't know how but I'm still here and you can see me." He stood awkwardly in front of Karkat not wanting to spook him even more.
"Prove it." Was all Karkat said. "I don't care how, if you are the real john just prove."
John looked around the room for inspiration, then he came up with an idea.
"Remember at my funeral when my dad was telling a story about the time me and my grandma played a trick on him but he couldn't finish the story?" he asked edging closer to Karkat who had calmed down slightly at this point.
"Yeah"
"I can finish it."
That first night the boys stayed up together all night, John finished the story his dad had started then upon Karkat's wish he had recounted what felt like his life story up until the point where he met Karkat on the first day at university.
"Is that enough proof?" John said to Karkat who was now sat cross-legged opposite him on the floor.
" For now…" Karkat replied slowly. "It sounds credible but you still could be a hallucination and I could have made it all up in my mind. For now I'll take it."
John let out a heavy sigh. "Well I can't do much else so I guess you'll just have to believe me."
"I've got to ask though." Karkat said thoughtfully. "Are you going to be here all the time now?"
"Well I can go away if you want, but you're the only one that can see and hear me, and I don't sleep, and I can't interact with any real world things so I'd like to say with you as much as I can." John replied quickly. "Cause the last few weeks have been really, really boring. Like really sad but also pretty boring."
Karkat let out a groan. Awesome he has his own personal ghost that nobody else can see. "Fine whatever. It's actually kinda nice to hear your dumb voice again. Just don't fucking watch me whist I'm asleep."
John let out a short laugh and it was actually pretty nice to laugh again, he hadn't realised how long it'd been since he'd actually had something to simile about. "Sure thing Karkat."