Yin is a battle butler who used to serve under his masters, the Harakiri Dust Corporation. Due to his intense training at a young age to be fit for them, he has a skinny, yet athletic build, while having a complexion that implies that he never smiles (which is very untrue).
He will almost always be wearing his battle outfit at all times, just in case of an ambush. His battle outfit consists of a black butler suit with white sleeves and a black tie, and a black coat over the suit. He also wears a black overcoat over the entire butler set with grey armor over his entire left upper limb. He has a bloodied rag tied over his right arm in memory of his former best friend and beloved Sonia Harakiri. He also wears metal plated gloves on both hands to try to avoid getting them cut off.
Whenever he (very rarely) is comfortable enough to feel safe, he will go into a black jeans and black undershirt combo for his casual wear. He prefers to wear black in his outfits because it is harder to look dirty.
Due to his interactions with Sonia Harakiri, Yin was able to develop his social skills where other people dedicated to their military training would falter. However, thanks to her death, he grew bitter to the point where he puts a cold and sarcastic front to deter others from going near him. If they were able to get past that demeanor and befriend him, he becomes very friendly with them to the point of seeming like a completely different person.
Because of the fact that he has little to no interactions with any of the lower class citizens, he is very curious about what they do. This curiousity can get the best of him when he doesn't know anything to the point where he can possibly become desperate or gullible to find out.
Yin is usually very uninterested in romance, having been very close to Sonia when she died, which mentally scarred him. However, his curiousity can get the best of him at times even when it comes to that. If he were to somehow fall in love with someone, he would get possessive whenever there was something that would take his beloved away from him, due to losing Sonia. If there are any signs that his beloved would leave him, he will get depressed and damaged inside, and try to think up plans in order to prevent any losses.
When in battle, mentally or physically, Yin's training takes full effect, as he becomes cold and calculating, discarding almost all of his emotions in order to accomplish his objective. The emotions that he doesn't discard are used in order to interact with others like when issuing orders to his teammates, or to speak to a witness. This can inadvertently cause his curiousity to leak out at times, proving to be detrimental to him.
Yin's main weapon is the Armored Heart. They are twin black swords (The Armor, and The Heart) with gun barrels on one side of both of them in their most basic form. Their hilts can shift and then either extend or shrink into the gun barrels so that they can combine to form a Spear, or they can also combine with Yin's armor on its forearm to form a Shield. All three forms of the Armored Heart can shoot bullets, thus giving Yin ranged forms of attack and repositioning skills.
Yin can also resort to martial arts when deprived of his Armored Heart, due to his battle butler training, but isn't proficient enough to make accurate attacks compared to the Armored Heart. His training also allows him to perform maneuvers that other people could not do, such as walk sideways on walls, but only for a limited amount of time.
His main weakness in combat is how mentally unstable he is due to the death of Sonia, and how much he values his friends over anything else, even his own missions.
Yin Yue was left behind by his parents at an orphanage as a baby. The only reason why people knew they were the ones to leave him behind was because of a letter from them that told of his name and why they had to leave him, which was because they lacked the resources to do so. They wrote in the letter that Yin was to be taken care of in the orphanage until their friends in the local Harakiri Dust Corporation would have enough time and room to take care of him themselves. The staff at the orphanage decided to wait to contact the Harakiris until Yin was able to properly take care of himself.
The orphanage staff watched as Yin grew up for about 5 years, and was surprised at his mental rate of growth. He was passing the developmental goals that were usually set for babies at double the rate. Nobody had any idea as to why that was happening, but they were thinking of letting him go to the Harakiris soon afterwards, especially when he made an act of bravery by rescuing some of the other children from a collapsing tree in the middle of a thunderstorm. Unknowingly to everybody else, this act was also observed by some of the butlers to the Harakiris.
The butlers soon arrived unnanounced at the orphanage, and were accompanying enough to give Yin a meeting time on the next day so that he could prepare everything he gained all those years, and then left. As Yin was preparing his belongings, he promised the rest of the orphanage that he wouldn't forget about them.
When the day arrived, Yin set off towards the local park that everybody in the orphanage would go to, and saw a legion of butlers standing there. As they saw him, the butlers parted to show a little girl in the middle of them who walked up to Yin and held out her hand. They exchanged names, the littler girl explained that her name was Sonia Harakiri, the heiress to the Harakiri Dust Corporation, and that she was one of the children who Yin had rescued in the thunderstorm. The two of them walked to the corporation building, where Yin was to become a butler for her, and then promptly began his training.
Yin would usually have his schedule made out for him, which was to wake up early, eating breakfast with Sonia, physical training like lifting weights, lunch with Sonia, mental activities such as classes, dinner with Sonia, and then playing with her until his bed time. There were some deviations in the schedule in times, such as to meet his higher ups in the battle butler Unit during Sonia's playtime, or to go on a mission. In time, Yin rose through the rankings of the Unit with the approval of most of them.
Yin and Sonia bonded extremely well together, and would always meet up during meal times. Excluding those times whenever Yin was forced to be in training, they were never seen apart. Even if Sonia fell ill, Yin would be seen by her side taking care of her to no end. Even when they were to be apart thanks to Yin's missions, they would often call each other via their cell phones. Everybody in the Corporation thought that they would end up getting married eventually. However, Yin only met Mr. Harakiri a couple of times throughout his entire teenagehood, and Ms. Harakiri had died giving birth to Sonia.
Years passed by as Yin continued to focus on his schedule, his training, and his relationship with Sonia, and soon at the age of 17, he was to be given a final mission to see whether or not he would be promoted to become one of the leaders of the Unit. However, he soon found out much to his great surprise that Sonia was to be married to an employee from a rival in the Dust Industry. As Yin questioned her in great distress, she kept on telling him that it was an arranged marriage, and that she had no say in it at all. Sonia was to make an announcement with her father to the rest of the company during a banquet a week before the wedding. The announcement that he was to be the security manager of the banquet for his final mission just made matters worse for him.
On the night of the banquet, Yin confronted Sonia and demanded for a full explanation soon before she was to make the announcement. She led him to a private room, told him to lock the door, and then turned away from him to look out the window to explain. Yin soon recieved several messages from his subordinates that the security up front was not responding and alerted Sonia. As she turned around to look at him, a gunshot echoed throughout the room, and Sonia fell down at his feet dead. As Yin tried to comprehend what was going on, the door burst open, and he was arrested by the rest of the staff on account of murdering her. He was to be given a private trial by Mr. Harakiri and some of the other leaders of the Battle Butler Unit. Yin managed to find evidence from Sonia that absolved him from guilt, and managed to pin the murder on an assassin named Indigo Yozora. He soon managed to find the person who had hired her to do the deed, but they managed to escape the room with the help of their own subordinates in the Unit, which erupted into a civil war. Yin managed to catch up to the culprit in the midst of the war, and successfully knocked them out, thinking that living in prison would make their suffering more unbearable than dying.
As the war ended, Mr. Harakiri decided to dissolve the corporation due to its divided state, but gave Yin one last request with his armor and overcoat as a gift before he went off to vacation: "Find Indigo Yozora. What you do from there is up to you." He suggested going to Beacon Academy due to its prestige and the status that the Hunters and Huntresses have.
A year after the banquet, Yin gets a letter from Beacon Academy thanks to Mr. Harakiri's help, and sets off on his quest to find Indigo Yozora.