Since we don't have more details about the light novel, we can only assume general things about the mc. The mc might be a person with a special ability or background that allows them to obtain and utilize liquid x. It could be that they have a unique heritage that gives them access to this monster - repelling liquid. Or perhaps they stumbled upon it during their travels and figured out its use through trial and error. The mc is the key player in this story who uses liquid x to fend off the monsters and drive the plot forward.
Well, without having read the specific light novel, it could be a number of things. It might be a magical potion that has properties which repel monsters. Maybe it's a special alchemical concoction that emits a scent or energy that monsters dislike.
It's difficult to determine its popularity. Just because we haven't seen it widely promoted doesn't mean it isn't popular among a certain group. It could be that it has a small but dedicated fan base that really enjoys the unique concept of the mc being forced to drink 'liquid x'. Maybe it's new and just starting to gain traction, or it could be an older title that has been overshadowed by more mainstream light novels.
I'm not sure specifically as I haven't read this particular light novel. But generally, if a main character (mc) is forced to drink 'liquid x', it could be part of some sort of experiment, a punishment, or a plot device in a strange or magical world where this liquid has special properties or consequences.
A mentor figure could be the one betraying the mc. The mc trusted this mentor completely, but the mentor has some hidden motives. Maybe they are working for a greater evil and see the mc as a threat or a tool to be used, so they betray the mc, which then causes the mc to turn dark.
There may be some novels out there that fit the general idea of a 'thrown away mc novel' that are popular, but without a more defined genre or more details it's difficult to name any. For example, some dystopian novels might have a main character who is cast aside by the ruling power or society, but they might not be directly called 'thrown away mc novel'.
Well, without more context, it's hard to say precisely. 'Thrown away mc novel' could potentially be a story where the main character has been rejected by society, family, or friends in the fictional world of the novel. Maybe the MC has some special abilities or qualities that others don't understand, leading to their being 'thrown away'.
Another aspect is prior knowledge or experience. In some light novels, the MC is reincarnated or transferred from another world. Like in 'Konosuba', Kazuma has knowledge from his previous life which he uses to his advantage in the new world, and this knowledge combined with his unique skills makes him overpowered in certain situations.
Yoosung could be a great addition. He could be the best friend who gives some comical or helpful advice. Jumin is also a good choice. His serious and business - like nature can create an interesting contrast to Saeyoung and the MC's relationship. And then there's Zen, who could offer some romantic competition or just be a good friend who supports them.
MC, who is the central figure through which the story unfolds, and 707, who usually has a special role. 707 might be portrayed as a very intelligent and resourceful character in the fanfiction. He could be the one with all the cool gadgets or special abilities that help MC and him get through various situations. MC, being the more relatable one, might be the one who provides the emotional core of the story, while 707 adds the element of excitement with his actions.
We can only guess about the main characters from this title. The mc is central, but there could be a whole cast of characters around them. There might be a character who created the medicine in the first place. There could be a love interest who is worried about the mc taking the medicine. And there could be a group of characters who are either on the mc's side or against the mc in relation to using the medicine, adding layers of conflict and drama to the story.