Choi So-Min was a rich pure-blood living off the wealth of her parents in Busan, South Korea. Yoo Kyung-Tae was a struggling muggle who couldn't keep up with bills. The couple had fell in love, however So-Min's parents did not approve of the relationship. And when she married Kyung-Tae her parents cut off her funding as she went against their wishes of marrying a rich man who was the CEO of a sister company to the Choi's. When she went against her parent's wishes, she had to find a way to make her own living alongside Kyung-Tae who was an unsuccessful lawyer. After losing their house when Kyung-Tae lost his job, the couple moved to Jeju Island in hopes they would have a better chance at living a semi-stable life.
For awhile things went well, they even had a twin boys named Yoo Chang-Hoon and Yoo Chang-Kyu. But once again Kyung-Tae lost his job which resulted in only one sort of income coming from So-Min's job. Despite the tough times, Chang-Kyu remained optimistic for his parents despite being extremely quiet. Their parents were close friends with a couple named Seo Seok-Ho and Seo Eun-Seo, the parents of twins named Seo Jung-Hyun and Seo Jung-Eun. The Seo couple was very wealthy and often tried to lend a helping hand during the Yoo's hard times.
When the children were four, Jung-Hyun and Jung-Eun's family decided to move to London, to engage their children in the rich wizarding community and wanted them to attend Hogwarts. This was during the time where the Yoo's were in a very tight spot and just couldn’t afford to care for both their sons. The Seo's offered to bring one of the boys with them. Although it was a heart wrenching decision, ultimately the Yoo couple decided to send Chang-Hoon to live with the Seo's in England, while Chang-Kyu stayed behind.
Chang-Kyu was never told about why Chang-Hoon left, not entirely. He was told he was going on a holiday with the Seo's and would be back one day. He never understood but it didn't stop the hope within him. Every year until he was 9 he patiently waited at the window on the anniversary of Chang-Hoon's departure, waiting for him to return. He never understood why his mother would threaten to burst into tears whenever he asked with an optimistic smile what time Chang-Hoon was coming home. He kept everything of his twins' pristine in the room they shared. What little things Chang-Hoon had left remained untouched, the only interaction Chang-Kyu had with them was cleaning them every so often. Keeping a serious of notes at the time and reflecting on his life at the moment, on his ninth birthday Chang-Kyu realised that Chang-Hoon wasn't coming back and the guilt of being picked over him was more than ever. Again, he couldn't understand why his parents were close to tears when he asked them if Chang-Hoon was ever coming back. Then, and only then, did he find out everything. Around them, glasses shattered as Chang-Kyu began to tremble. Fearing the worst, his parents ran out of the room and left the distraught child there.
After that, Chang-Kyu closed up. Albeit he was never loud in the first place the whole situation cut him off from the world socially. Instead, he dedicated his time to learning English - excelling in the subject at muggle school. It became his mission to one day reconcile with his twin who he referred to as Caiden upon learning the other's new identity. On his tenth birthday, his mother handed him a large sum of money for his birthday, telling him she placed him on an exchange student to England. When he inquired about the money she simply smiled sadly and told him to never return to Jeju without Caiden. She told him to stay in England and never come home until he reconnected with his twin. Later that day, Chang-Kyu found himself on the plane to a house in the West End of London.
His new family were kinder - they tried to help him open up socially but it never happened. Chang-Kyu, now called Luka, remained silent as silent could be - slowly showing the tell-tale signs of anxiety and the beginning stages of depression. Only once did his mood brighten among the pure-blooded family he was with and that was when his letter to Hogwarts arrived.
He's been at Hogwarts for five years now - he's reconciled with Caiden and unlike his daydreams, it didn't make him fully confident like he had hoped.