At the beginning, Yancheon adopted the same typical passive behaviour we see often; I dont want to get involved with anyone, everything is a pain in the ass, I want to live a quiet and peaceful life, even took in consideration moving to the mountains and live in reclusion until everything is over with the demon war and the hevenly demon since this is defeated and killed by the zenith and now current childhoodfriend, Wi seolah. However, slowly but surely he changed his mindset and start taking responsability on his duties as a regressor.
The MC is not your common doormat nor pushover. If some as*hole attack him or act in any disrecpetful, impolite or offensive way, he will retaliate with everything. By far, the MC have killed a few ones, and amongst those few, would have been young masters of others clans, who were saved just because someone has been there to stop the MC.
But at the same time he showes vulnerable sides that make him human and not the perfect and HANDSOME guy usually seen in the korean standards. For instance: when his father praised him for rising to another realm, his lips almost curled into a smile, even though he has the mentality of a grown up adult, he never received a compliment for his father, so he was subconsciously happy

The depictions of the relationship with his family and relatives is so complex and interesting, I am never tired of it.
He regards them, his family, as a bunch of battle maniacs who only knows how to use their punches to comunicate and finds them annoying, yet he cares for all of them and feels grateful because they stood all of his sh*t throughout the years.
The brute but caring older sister Huibi and the mean and playful Second elder are my favorites.
Not to mention the heroines. Wi seol ah, the childish but misterious future zenith, the beatiful and enchanting but nonchalant and cool Bi Ah, and the unique poisonous queen, soyeol. Any of them are in love with the main characters, are attracted or interested in him for various reasons.
I like this, how the author is building the relationship with the MC slowly and not introducing the romance so early.
Also, I Have read several complaints about how he is "dense", and does not reciprocated the feelings of the heroines or ignored their advances. Yancheon is not dense, rather he has a guilty conscience. We have to understand he was involved in their deaths, except for Seolah. In adittion, the girls are not the most expressive; Seol ah is just a kid, Nangung Biah is following him for his lack of smell, how is she going to explain this? And Soyeol finds his scary and sharp facial features attractive. He has not done anything to gain earn their affection, so, is pretty normal he is bewildered by their sudden approaches.
As an extra, I just love how in every illustration in which the heroines appear, they dont take suggestive poses making emphasis in the breast, butts and thighs, to say the least. In other words, the author doesnt appeal the horniness of the readers. This show how much he repects his characters and his readers.

I dont care how many power ups, skills, abilities or realms he get as long as he does everything by him own will and overcome hurdles or hardships in his way to prove his repentance is genuine. This is very important because what made him deserted his clan and joined the demon army was his inferioty complex of being compared to his two talented elder sisters and he being a arrogant and ignorant shitty brat who never put efforts in anything.
The MC beat the sh*t out of his cousin and the nangum young master using his experience, perfect control of ki and taking advantage of them looking down of him, he literally swept the floor with them by training and putting effort. By far pretty good.
He also got the power of the golden nature treasure which increased his capacity of Qi and helped him raise to the 3rd real, almost 4th, located in sichuan where the tang clan resides. I has no problems with this, since this was planned from the very start.
But, after that, he found out he still has the ability bestowed by the heveanly demon, demonic absortion by pure coincidence. This allowed the MC spam like crazy his flame martial arts and consisted in absorbing the demon energy from others users. But absorbing a lot of demonic Qi makes the users goes crazy and addictive, so he cannot abuse from this, right? No. Also, He found a special trait who purified and assimilated the demonic Qi to his flame arts. He even was in shock because this ability never manifested itself in his past life and there is no records of this in the Gu clan. Wasnt he talentless???
Later, he accidentally run across the sacred treasure of the plum blossom sect, absorbed its Qi and gained a spiritual master who will save his ass subsequently. A fortuitous encounter. Step by step he becomes a overpowered guy.
Stick to your ideas until the end. If you are gonna give me a main character who main reason to desert his clan was his powerlessness, then dont make him the most lucky guy by gaining random powers constantly.

I said that I like the the depiction of the interactions between the MC and the secondaries; but, apart from that, there is nothing interesting in them since they do nothing for the sake of the plot.
For instance, In the plum blossom sect, Yancheon killed and chased away the intruders, saved his little sister and Biah, killed the branch manager of the black palace, discovered the corpses of the disciples who dissapeared, killed the assassin send by the plack palace, cured the plum blossom master who was suffering who even the inmortal healer could not healed.

And where were the others doing? I guess, they were poking their noses and scracthing their balls.

The secondaries characters are the ones who make the world that turns around the MC alive and attractive with their interactions with the MC and each other, and their actions impact the plot.
So, If you are going to introduce so called Geniuses, prodigies, and master who can crush mountains, make them do something worthy the title. The fact, they are all smokes and mirrors.
If every final decision falls in the MC hands, your world is dead and boring.
Even BiAh had to be saved as f*cking damsell in distress by the MC at the exact moment.