Other People's Traits

Sans' traits

Physically, he shows off his blue eyelight and red scar on his chest along with the skeleton appearance he has. This is due to Sans being the only 'vessel' properly available in the moment of the accident, better called 'sprite'. Personality-wise, Rain can be quite smart because of him. He is not exactly bad judging people on first sight, despite trying to be openminded or skipping over bad thoughts, but he can't really avoid feeling distrustful because of Sans' personality. It gives him a heads up sometimes when it comes to 'odd' people, sometimes even getting to act more aggressive if there is actual danger. When his personality leads to that extreme, he is even capable of attacking in self defense or by being triggered. But Rain being a pacifist, he will always try to stop himself or those impulses sabotaging himself as he physically can. 'This' Sans, after the Genocide timelines, became very short tempered and grumpy, keeping a deep sadness and hollowness in his soul.

Papyrus traits

Physically, his orange eyelight and most of his height are what he got from Papyrus along with the red scar along his neck, as Papyrus was barely a physical influence at the moment Rain was created.Personality-wise, Rain gets most of his compassion and beliefs from Papyrus. Papyrus was forgiving until the end of their messed up world, even after being betrayed, murdered, and provoked to fight back to eventually give up to his kindness on the Genocide Routes. Rain wants to believe in others no matter what their circumstances were, or how they behave. He truly wants to hold onto the idea that 'Everyone can be good if they try', feeling like that's the actual answer to dealing with troublesome people. Without Papyrus', Rain's existence would have been way more of a mistake, since he would have somehow self destructed out of how incompatible the other two people are to be 'stuck' together. Papyrus is what actually keeps the 'Determination' to live with his own being, not allowing them to give up and trying to help them remember that they must be kinder to themselves.

Human traits

There is not too much influence from their part. The human is what keeps their soul together with their determination. After the Genocide Route, Frisk stopped being determined. They didn't think they had any way out of that mess once it had happened, but at the moment of the accident, they felt determined to keep them alive, and that is how their souls ended up figuring out a way to hold each other in a sustainable way where they would not disappear. The other thing Rain can notice from the human is the permanent guilt and regret coming from their soul, which, while it doesn't hurt or makes him sad all the time, he can notice without much effort.

Relationships

Insans (by Tatatale)

Insans became his adoptive brother after he had found Rain on his world. They started hanging out, and eventually given Rain's resemblance to Papyrus, Insans decided to help him out with his basic needs, company, socialization, and eventually with his abilities. Insans became for Rain someone very important which he treasures more than most people, to the point he can be very hypocritical excusing his 'brother' issues as they clash against his own beliefs. Nowadays they live together in the same house, sometimes helping in Insans boyfriend's restaurant and all.

Cheshire (by Alch3mic)

Cheshire after some silly and tense circumstances, became best friends out of a joke. This eventually became a fact, though. Rain really appreciated him even if he was able to acknowledge that it started being not serious, but little by little with time the caring for each other became mutual not minding their differences in likings or activities at all. They are able to be comfortable with each other despite those facts, and try to understand each other's oddness to the point they can hang out casually and have fun like the good best friends they became.

Others

Rain feels a deep affection towards all the monsters in the Underground, and because of this he also has developed a great fear or guilt / concern about meeting 'again' some of them, especially Toriel, Asgore, Sans, Papyrus, Undyne, and the amalgamates.Sides to this, he has met some other friends or questionable friends on his travels, that might or not be added in the future.

Backstory

Long time ago, a human fell in the Underground.They were scared and lost, but determined to get their way out, so they started their journey through the Underground. They quickly got attached to some monsters as well as scared of a few others, but didn't take long to realize they all were as desperate as them to get out, but there was very little that this human could do to help. And eventually they died. But, they also came back. Unaware of this ability, the human tried to gloss over it and continued their way out of the Underground with some struggles, until they were free.But what would happen to those kind monsters if they were left behind like that? They could not fathom the thought of them being in despair for much longer...And so, they went back to the beginning.

They started to figure out how their new ability worked, an ability called 'DETERMINATION' as they learned, the ability to persist after death, and the ability they would use to save them all. The human tried and tried, befriending everyone they could not before, learning more about them, growing interested in their lifes, until they indeed got to the end once more, this time with everyone, a True Pacifist route.They were set free.Or, almost everyone was set free. It was a very nice ending, for sure, but not everyone was saved, or was it? Asriel was not saved, á̸̦n̶͙͝d̷͉̈́ ̶͈̈n̷̼̅ë̵̻́i̶̬̊ț̸̓ḧ̴̙́ḙ̴́ṟ̷̛ ̵̩̏w̷̘̕a̵̜̒s̵̬̽ ̷̡͠G̷̬̚a̷͇͝s̵̠͒t̶̞̍ẹ̵͒r̶̠̂.̸̟̒So they decided to go back again. And again. And again. And again. They kept trying different routes, they tried to talk more to them individually, they tried different approaches in the fights, they tried different endings, but nothing got them to reach this 'perfect' ending.Instead of what was expected, something that had been growing angered was being awoken after each reset.And one day, the human got to their first kill, by accident. It was alright, though. They could just reset back to the beginning like nothing had happened, but... The more they became distant with reality with each reset, the more they had messed up to the point of no going back. Something took over them, and genocide routes started happening. Something was not right with them, either. They were cold and cruel. They got rid of all the 'boss' monsters, they got rid of everyone who they had not befriended, they got rid everyone who they had befriended, everyone who mattered and, or could have mattered to them.They completed a True Genocide route.And little by little the human's Determination started to fade, hurt by all the damage done that they got reminded they started for trying to reach an impossible happy ending. At the end they had helped no one, and they brought them despair themselves. Even worse, they brought death upon them.BrokenThe world was far from being saved as well, growing corrupted by each reset. U̵n̵t̸i̸l̶ ̶t̵h̶e̵ ̶g̷a̵m̶e̵ ̵c̷r̷a̴s̸h̷e̸d̷.̸ And the human disappeared right before killing off all the boss monsters in the Underground, the amalgamates, and all the normal monsters as well, or mostly, saving a last battle until the end in the judgement hall. Sans waited for the human, but nobody came. And nobody appeared for a long time, either. Until one day, they did.The human appeared again on the beginning, unable to reset or use their determination to do anything at all, but they could still talk, couldn't they. So if they figured out a way to fix this world and bring everyone back, they could reach a happy ending for good. But they did not count with the fact that the remaining other one was way too exhausted to follow their thought thread.They barely wanted to face the human, but they were somewhat desperate to see a living being again after so long. Only because of that they decided to go along with their rambles and follow to the CORE.It was absurd.
Everything the human was saying sounded absurd to him, especially after everything they had done, how could they forgive them? And they dared to mention his brother again, becoming what cut his last thread of self control remaining.The human tried to bring back his brother as a proof, but Sans had quickly grabbed the human and decided what was a better ending for this all and for them, an ending that he had been waiting so long for, so he threw himself and the human down to the CORE. What they did not expect is that the human had succeeded in bringing back another confused skeleton. Papyrus with his little bit of reasoning only managed to reach out and hold his brother's clothes, but as they all were in an awful position, they all quickly fell down. A̵ ̷l̴a̸s̶t̶ ̵r̷e̴s̷e̸t̴ ̵h̴a̷p̷p̷e̵n̵e̴d̴.̷And they woke up.