The main character Sirius, unlike most generic and dense protagonists, is very capable, intelligent, and is self aware in regards to his current situation. While the supporting cast introduced (Elena, Noel, and Dee) are heavily riddled with cliches, they all serve a certain niche in the story that makes them tolerable and relevant to the narrative, especially Elena. As the story progresses in its early stages, Sirius' growth and discovery of the world around him made the story genuinely interesting. Furthermore, when new characters were introduced, Fia and Lior, their interactions with the protagonist were engaging and natural.
However, the quality of the story takes a noticeable drop after Sirius gets his first disciples. Sirius, while self aware and intelligent, becomes a perfect definition of a Gary Stu; he can solve everything with little to no effort... wait, Sirius claims he 'works hard' for it, but the author just tells you that because he/she is too lazy to write actual scenarios where the readers can empathize with the protagonist's handwork. Instead, Sirius can cook, cast very powerful 1hko magic with no incantations, beat legendary sword masters, draw magic circles that are complicated to even magicians, and seduce 50% of the female cast at the age of five.
Remember when I said the main supporting cast was tolerable and relevant? Scratch that. Everyone in this story, other than the antagonists, seem to have their mouths on Sirius' nutsacks all the time. Everything the main character does has to have some echoes of admiration and amazement from the other characters; essentially turning the story into a one massive circlejerk for the protagonist. After a certain number of chapters, not only do the supporting cast not get any development, they get reduced into one dimensional characters. I said one paragraph ago that I enjoy the interactions between Sirius and Lior; few chapters later, he gets reduced into a dumb, loud, battle maniac with muscles for brains. The female disciple, forgot her name, is just there to constantly (I mean CONSTANTLY) compliment the main protagonist to further solidify his awesomeness and give him a potential love interest. The other disciple, Reus, started out as an annoying brat who disliked Sirius. He then goes through a character arc and later develops to become an annoying brat who worships Sirius and, you guessed it, compliment how good Sirius is.
The antagonists, while they certainly do not share the same admiration of Sirius as the supporting cast, feels like the author inserted them there not to provide any challenges for the protagonists, but to provide some juxtaposition and to emphasis how awesome Sirius-sama is. Everything in the story is black and white; the protagonists are holy people and the antagonists are pure scum of the earth. Even though Sirius literally shoots someone in the head, the act of murder is painted as a righteous deed because we do not feel any connections with the bad guy (he is bad just because).
To give credit where credit is due, a character arc involving the side character Elena actually provided some heart to the story and some touching moments (but the author fails to capitalize on this pivotal moment and develop the characters in the story). Also, like I mentioned before, the early chapters were enjoyable.
Overall, the story is nothing new (typical isekai), the side characters (except for Fia and Elena) are horrendously written, one dimensional, and intolerable due to their constant admiration of the main character, and the main character is just a massive wish fulfillment for the readers who solves every problems effortlessly. I am not hating on the novel; in fact, the reason why I wrote this rant is because I genuinely enjoyed it early on and had high hopes for the story. But that's just my subjective opinion and I hope you may be able to form your own while reading the novel.
