Mashiro

Funfacts:
The ball on the end of his tail can glow so it's like a latern light
His nickname is Mushroom-Senpai. Shuichi called him that to make fun of him. Senpai is ironic of course x)
He can be drawn with all sorts of animals outside in nature.

Story:
Mashiro was born a normal Tsunoka. He used to have two horns where the mushrooms are now, no moss on his body, white hair without the other colors, no animal ears, purple eyes etc. It's unclear what kinds of abilities he would have if he had grown up like this. However he became a test subject, which changed everything for him.

When Mashiro was a little child, he lived a happy life in a normal family. he had a mother, a father and an older brother. And he was very attached to that brother. They were always together. Mashiro was a nice and happy child. You'd often see him smiling and laughing, trying to help others and so on.
However somewhere else in the region he lived in there was a scientist called Shuichi, who was doing research in his lab. At this point in time he was trying to find out more about magic, thus he was researching the horn of a tsunoka which contained magic. And he used this horn to infuse some kind of drug he created with magic to make it more efficient. He miscalculated greatly on this one however and the drug turned into a gas and spread. As it turns out it developed poisonous characteristics as well. It multiplyed with the magic power of the horn and the gas spread outside the lab. So fast, when Shuichi disconnected the drug from the horn, it was already too late. Everything around had become infused with the poisonous gas and it spread further and further.
Shuichi had a special resistance against all sorts of poison so he did not notice what exactly was going on soon enough. But when he left the lab to see what had happened, he noticed everyone around had dropped dead. Panicked he tried to warn people further away through his communication network to completely evacuate this region. Some people made it however most became victims of the poison.

After that Llyr, the creator of this world paid Shuichi a visit, lecturing him about what had happened. He wasn't happy about what had happened, however he knew it was an accident and thus he decided to forgive Shuichi and to leave this region (which is a seperate dimension in this world) to him, so he could continue doing his research there without running the risk to harm anyone ever again. After all it was now uninhabitable land.
So Shuichi went to do some research on the poison that spread, he walked through the region to observe the effects. He found lots and lots of corpses, dead plants, contamined water and so on. Some plants managed to survive and build up resistance. He also encountered some animals that he never saw before. It seems the poison had an enhancing effect on them rather than a deadly one.
And then... He encountered Mashiro. A little boy who was desperately trying to make one of the corpses drink water, a brown contamined water... That was all that was available. The corpse was the now not recognizeable older brother he cared so much about. He did not realize he was dead. He thought it was an illness, something he could fix.
Shuichi was astonished. Mashiro was the first person he found, who was actually alive after the incident. He decided to take him to the lab, do some experiemts and possibly stabilize him, in case the poison might have an effect after all - even if it's not a strong one. So he went and told him his brother was dead. Mashiro, a naive little child at this point did't know what dead meant and his eyes started to light up. He asked Shuichi, if he could fix that. Since he knew the kind of condition his brother had. So Shuichi explained. And Mashiro fainted from the shock.
Shuichi carried him to the lab and started investigating his body. He found his immune system to be much stronger than those of the other people. However, his body was not strong enough to withstand the poison for much longer. He was already becoming weaker. So Shuichi tried a chemical on his body thinking it'd help him. But it turned out this chemical changed its properties thanks to the poison. Moss started growing on Mashiro's body one of his eyes became green, his horns turned into mushrooms. The poison itsself was not the problem anymore, however the moss was a completely different issue. If it were to spread too much he'd die.
Shuichi did his best to make an antidote with the limited things he had - most was contamined by the poison after all. And he managed to make a pill. This would stop the moss from spreading further. Mashiro would have to take one every 24 hours. (These are the pills in the bottle, he usually has them in the pocket of his pants) Sadly the side effect would cause him to be tired 24/7

When mashiro woke up he felt weird and he was wondering what was going on. He was very sleepy, but he listened. His first conclusion was that Shuichi was his life savor. That he was a good guy helping him. The moss thing was an unfortunate accident. However then Shuichi got to the point where he explained the cause of everything and Mashiro realized, that this was the person who caused everything to begin with. The death of his family was entirely his fault. Unable to think clearly he tried to kill Shuichi - didn't work. He started a few attempts after that but eventually gave up on it. Mashiro discovered his ability to produce poisonous gas... Not that it'd hurt Shuichi tho. In the end he gave up on his revenge, realizing that he had no chance. And that Shuichi as much as he hated it was his only way to stay alive now. Nobody else would be able to produce his pills.

Over time due to the effects of the pills he became to tired for strong emotions. And after giving up assassinating him, he also gave up hating Shuichi. Mashiro started assisting Shuichi in the lab. He hated it, however he was able to find out more about the poison, his condition, the ways to fix it and so on. He became a fine assistent for Shuichi despite his attitude towards him.
His conditions improved after they worked on a better pill. It wouldn't make him as tired anymore, however his indifferent character did not change back to the once happy child. He still found sleeping to be his favorite hobby.
Another thing they tried was fusing Shuichi with one of the animals who had improved abilities rather than declining from the poison. That sadly did nothing to improve his condition, it gave him animal ears with enhanced hearing (which kept him from sleeping) and a tail that could glow. Generally rather backwards. However it caused Mashiro to look how he does now.

This is how they went on for a while until Shuichi let Mashiro go to another region for the first time. The region where his friends live. Mashiro was shy and didn't fit in very well. He didn't like being around them. However he discovered a forest. And he loved spending time there with the animals living inside. They liked him, he liked them.
He finally had found a peaceful place for himself, a place where he could enjoy himself a little bit.