Anima Requiem: I Bought a Katana Online and Triggered the Apocalypse - Chapter 15: World Greatest Delivery Service

Every novel has a slag character, the one who decides to abandon the protagonist at his lowest, most helpless point, either because he can’t stand the poor guy’s circumstances or for other reasons. But after the protagonist wins his way in life again, they’re the ones who beg with broken sobs to be taken back.

Still, water once spilled can’t be recovered again.

Yet the unlucky Shen Jiayan has been bound to a Water Recovery* System to do just that.

*water recovery – comes from an idiom that means “spilled water can’t be recovered” (English equivalent: there’s no use crying over spilt milk).

- Description from Novelupdates

I died a virgin right after winning the lottery. Bad luck? That's my brand. So when a Grim Reaper offered me reincarnation with a gacha system for a “second chance,” I should've known better than to trust my luck.I rolled... an orc. The only fat orc in my entire tribe.The dungeon's demon lord gave me three years to lose weight or die. I trained harder than anyone, became stronger than most warriors in my tribe, but my belly stayed huge.With hours left to live, I made the dumbest decision of both my lives: if I'm dying anyway, might as well lose my virginity first.[Virginity Lost! System activated!][Obtain dungeon control upon complete subjugation]Turns out I was supposed to be a DUNGEON MASTER, not a slave warrior. And turns out I've committed the ultimate taboo: interspecies breeding, forbidden across all civilizations. Now I'm a fugitive.Humans, demons, and monsters alike want me dead. My own kind thinks I'm disgusting. I have absolutely nothing left to lose.Time to turn the worst gacha pull in history into the greatest dungeon the world has ever seen.

Our protagonist Caam is a man from Earth that was reincarnated into this world as a demon. In order to survive, he spared no effort in avoiding the sparks of battle and without noticing it, he became strong himself.

In novels with a stud there is always a Senior Martial Brother like this:

In the early stages of the novel, his innate talent is high, his cultivation is good, his appearance is handsome, and he has an intelligent, cute younger Martial Sister as a fiancée.

In the middle stages of the novel, he is caught unprepared and defeated by a young disciple (the protagonist) in the sect who’s hardly remarkable at all. From then on, he falls from his godly altar and loses his brilliance. Even his young Martial Sister leaves him for someone else.

In the late stages of the novel, he continues to turn dark, going down the Demonic Path and becoming the major villainous boss. However, the protagonist still ruthlessly defeats him and his soul is eternally sealed.

Once Qin Mo opened his eyes, he discovered that he had become this Senior Martial Brother.

- Description from Novelupdates