Poster credits: MusicChibi from King for a Day Graphic Shop

This is not necessary to read but I compiled a glossary for those who might not understand the meaning of certain Korean words I will use through the story. It would be easier to understand the characters and their relationships with each other if you understand the meaning of the terms below and the implications that come with the usage.

For those who are unaware, there are certain styles to speech for Koreans, rather like the Japanese. Language and manners are very important in Korean society and the usage of incorrect terms during conversations could actually result in the other person taking offence. I do not speak or write Korean, and any informed reader is welcome to correct me if I'm wrong, but I have a very simple understanding of the hierarchies and basic terms that are present in speech.

There are strict rules to adhere to in speech for Koreans and what form of language to use which mostly depend on someone's familiarity with their partner. Banmal is informal speech, which is used for people you are familiar and close to. It should only be used with close friends or younger people – like an elder to a teenager – but if a younger person or someone unfamiliar were to use banmal, it can come off very rude. Also, sometimes dropping to banmal could happen when people intend to offend. 

Jondaemal is the polite form of speech people use when meeting strangers or people they're not necessarily close to. Younger people also use jondaemal with some relatives or friends who are older even if they may have a good relationship. It depends on whether the older person is more relaxed and gives the younger one the go ahead to drop to banmal. Jondaemal is much more formal, and may indicate a distance between the two speakers. Honorifics – suffixes like -sshi and -ah – reflect the speakers' relationship and differentiate between formal and informal speech as well.

Glossary:

Abeoji: a formal term for father.

Agasshi: miss.

Ahjumma: middle-aged woman who can be a stranger, or a close friend of the family.

Ahjusshi: middle-aged man.

Appa: an affectionate term for father, like the Korean version of dad or daddy.

Chaebol: a family-owned conglomerate of businesses. Also used to refer to the members of the wealthy family that runs the conglomerate. One particular website described a chaebol's English equivalent as a tycoon.

Dongsaeng: younger sibling, or a younger close friend whom you treat as a sibling. Not gender specific, unlike noona, unnie, hyung etc.

Omoni: a formal term for mother.

Omma: mom, mommy.

Halmeoni: grandmother.

Harabeoji: grandfather.

Hoobae: used to describe a junior, be it a junior colleague or someone from school who is in the grade below yours. It doesn't necessarily mean that person is younger, and is just a term, so people don't address the other person as hoobae. It's more like, "That's my hoobae, Nam Taehyun. Yah, Taehyun ah, come here and meet someone!" See?

Hyung: used by males for older men. Can be used for siblings, or close friends. Hyungnim is a more formal version of hyung, like a younger man with his brother-in-law etc.

Hyungsunim: older brother's wife.

Jageun abeoji: father's younger brother. Paternal uncle.

Jageun omoni: wife of father's younger brother. Paternal aunt.

Keun abeoji: father's older brother. Paternal uncle. Literally translated means big father.

Keun omoni: father's older sister-in-law. Paternal aunt-in-law.

Maknae: youngest in the group, or family.

Noona: used by men for big sisters or close female friends who are older.

Oppa: older brother. Can also be used for an older male friend who you're close to, or even boyfriends. 

Oraboni: a polite, more formal version of oppa.

Sajangnim: used to address the boss or a company's president.

Seobangnim: husband's married younger brother.

Seonsaengnim: a respectful way to address a teacher, or even used by the speaker to address an experienced elder working in the same field whom they deeply respect.

Sunbae: means senior. The suffix -nim would make it more formal.

Unnie: used by females for biological older sisters or close friends who are older.

Honorifics: 

-ah: is used when the speaker is addressing someone affectionately in an informal manner, e.g. "Seunghoon ah!"

-nim: a suffix that connotates respect, and is often used when addressing someone much older or experienced. However, its absence does not indicate a lack of respect. Someone may call biological brother hyung, but call his friend's older brother hyungnim instead.

-sshi: used between people of equal status in formal speech. It is used after the full name such as Song Mino sshi, or simply after the first name like Mino sshi if the speakers are more familiar with each other. However, using it right after the surname, such as Song sshi, might be rude, as it may seem that the speaker is putting him or herself on a higher social level than the other.

WARNING: This is a heads up for readers who might get upset over how their biases are potrayed in the story. Many characters will do reckless, horribly immoral things too. Take note, I don't mind doing anything drastic to the characters.