My Demon

Hell-Borne Soul Hunters:
Both feature actual entities from Hell (a demon and an underworld judge) who walk the mortal world to harvest or punish the souls of corrupt humans.
Glitched Supernatural Powers:
Both powerful leads face a sudden loss of their absolute abilities after interacting with a specific human, forcing them to experience mortal pain and vulnerability.
Chaotic Enemies-to-Lovers Chemistry:
Both shows thrive on a fiery, bickering relationship where a ruthless supernatural predator is forced into a protective alliance with a human, eventually learning empathy.

Mermaid Got Married is the Splash-like tale of a beautiful mermaid who loses her pearl when she saves high school teacher Dior Cheng from drowning. It turns out she also saved him when they were both kids so there's an immediate connection between the two. Unfortunately, the pearl is the source of her power so she follows him onto land.

I got that one shows hetero couple and not for the other…
But the first thing I realize is the fact that both Demon and Vampire in those series lost their power and incapable of doing some sort of amazing action… ??

The plot looks similar
Both the demon and the gumiho will turn into a human after meeting the leads and both the demon and gumiho is a narcissistic and refuse to become a human
Not sure but I think this couple will also have a past life drama

Both of these dramas main plot point is that the ML is a 'demon'. In both dramas the ML who is a 'demon' makes binding contracts with people and comes to collect at some point. In both dramas this ML who is a 'demon' falls in love with the other lead who has an innocent quality to their character. And in both dramas this lead can't help but love them even when they know they shouldn't. Besides all of this, these dramas are drastically different. So you get a whole fresh story with fresh plot points which is great.

A baker by day and demon fighter by night, Zhong Kui, a reincarnated deity must jog his amnesiac lover's memory of their millennium-long romance.

Hua Xiao Xuan is a witch of a supernatural flower shop called "The Memory Garden," where she helps her customers let go of their fixation and move on with their lives. One day, she meets the man who accidentally stole her sense of smell during her 13th regeneration years ago. The sense makes him immune to her powers and allows him to enter her shop without invitation. She foresees their lives intertwine and the darkness she is fighting against may cause his death someday.

The fate of these people become twisted due to "hwanhonsool" (the soul of the dead return to the living). In the country of Daeho, Jang Wook comes from the noble Jang family. He holds an unpleasant secret about his birth, which people all around the country talk about. He's a troublemaker. Jang Wook happens to meet Mu Deok. She is an elite warrior, but her soul is trapped in a physically weak body. She becomes Jang Wook’s servant, but she also secretly teaches him how to fight. Seo Yool comes from the noble Seo family. He seems perfect with good appearance, intelligence, and strong martial arts skills. Go Won is the crown prince of Daeho. He hopes to become a generous king

Follows an arrogant, wealthy heiress who enters into a contract marriage with a cynical, powerless demon to mutually protect their interests, before real feelings inevitably get in the way.

Jang Uk returns from death, and three years later, the story of the mages unfolds anew. Jang Uk becomes a hunter of the soul-shifters when a young woman, a prisoner in her own home, seeks his help to reclaim her freedom.

Same demonic atmosphere

The actor has a song kang vibes

The guy chases the demons but the presence of them make the drama similar

En mi opinión, son similares a la hora de juntar la vida real con la fantasía, y también en el sentido de que en los dos dramas, los personajes están en busca de algo.