This mini-chapter is to give a little background to Emerald and Mizukishi in a "how they got to where we are now" type of story.
Emerald Acid is a character that belongs to me, and Harusame Mizukishi is my friend's creation that I am using with his consent.
All other characters, as well as the Touhou universe, belong to ZUN.
Emerald was raised in the human village by his mother, Spinel. He was raised in a bit of secrecy as his mother dressed him with a hat to cover his antennae and a cape to help hide his wings that could sometimes be seen through his loose fitting top clothing.
He was also brought up as if he were a normal human, the only people knowing otherwise were himself, his mother, the village elder, and the Hieda family. The elder, knowing their circumstances, had no issues with allowing Emerald to live in the village as a human.
One day, he was out in an open field on his own when he encountered Harusame Mizukishi. Mizu approached Emerald and immediately identified him as a mixed-blood youkai as for all intents and purposes, Emerald is classified as a youkai due to being "upgraded" from a half- youkai due to the incident.
As Emerald attempted to hide it, Mizu told he had no ill intent and even revealed he was a youkai as well; a pure-blooded Magician. Being a youkai hiding as a human as well, Mizu was interested and wanted to reach out to Emerald for being in a similar situation.
The two talked for the remainder of the day and easily became friends that would remain as such even up to the current time in Gensokyo.
Shortly after his 16th birthday, his mother passed due to complications caused by her injury, leaving him alone aside from his friendship with Mizu. The village elder supported him by means of funds to live, but Emerald was mostly left to himself due to being reclusive from his upbringing.
Two years later, Emerald stopped aging visibly. It wouldn't get noticed anytime soon, but in the following decade or so, some people became suspicious. The majority of the conspirators eventually accepted that the trauma of his mother passing so early was having an effect on his body.
Another decade later, and both Emerald and Mizukishi would be exposed. Another incident was happening, which had drawn the attention of the next generation of the Hakurei Miko. The village again came under attack, but this time only by weak, small time, yet still man eating youkai.
This generation of Hakurei was a bit more gung-ho than her predecessor, and had raced off to find the source while not bothering to even check on the village, and at the time most of the fight-capable village defenders were out fortifying some defensive outposts outside of the village.
When the attack came, Emerald and Mizukishi would realize that, while weak, the attackers would overrun what few defenders were left, and upon request from the village elder, took to the front lines. The two easily cleaned up the attacking youkai, finishing off the last of them as the defenders returned.
By saving the village though, the two had been officially discovered as non-human. At first and for a couple months to follow things seemed normal, but eventually the human's natural paranoia of things not of their species began to whisper accusations at Emerald and Mizukishi of secretly orchestrating the youkai attack.
Eventually, the accusations grew venomous even towards his deceased mother for birthing a "creature" like Emerald. Unable to stand it anymore Emerald left the village and eventually came across the tree that he would build his house on.
Mizukishi had fallen off the radar at this point as a conspirator due to Magicians being much closer to humans than other youkai, but after seeing how the villagers turned on Emerald despite his actions to protect them, he left as well as to not deal with them, although he still occasionally visited the village for supplies and to see the elder.
Over the years, Emerald would develop his house into the multi-tiered dwelling that it currently is, and mastered the practice of using the Forest of Magic's naturally infused materials in weapons, and settled on a bow to play to his strengths such as his powerful eyesight, natural affinity for range and "thousand cuts" style.
A generation passed, and the powers of the Hakurei Miko would fall onto Reimu, bringing us to the present time. Emerald had met a handful of other people, mostly youkai, and had even made a few friends amongst them.