Both dramas deal around the concept of artists that paint and help solve crimes with their skills. While Sketch has a supernatural aspect to it and Under the Skin focuses on a forensic artist, they both are very interesting and follow unique concepts.

Basically the same story but different settings. We have an age gap romance where the woman is older, both couples end up working together, in both dramas one of them is the boss of the other, and many other aspects that would be a too huge of a spoiler.

Both dramas are about an amnesia story where the female lead finds out she is married/the fiancé of a guy. Said guy tries to be understanding and helpful of the girl, however something seems suspicious about him. Both have very well done story twists that one does not truly see coming.

Both dramas deal about a girl in college that has a tendency to kill people, who did bad things. And in both dramas she meets a guy that helps her and knows about her tendencies. Both are very similarish with how the girl works and who helps her.

Tragic storylines, with the main focus being on profiling. Especially the first couple of episodes of Crime Puzzle are quite similar to Through the Darkness. Also both male leads are quite similar in demeanor.

Both dramas are about profiling.
Both are taking place in various time zones starting from the 80ties to the 2000s.
Both are thrillers with interesting cases. Both form teams that specialize in this.
And both have a male lead that is shaped due to his childhood and is doing a sort of narrative analysis's throughout the episodes.

Both dramas deal about infidelity and marriages being tested. Both show the struggles of a woman, who has to find her way out of a huge gap that opened up.

What is justified when it comes to justice? Both dramas thematize this question. Both have criminals and the question of how far the legal system should go in order to punish them. Both demonstrate good and evil, morals and fairness. Both main characters go controversial ways and make you question your own standpoint on how far one can go for it to be too far.

Both dramas deal about crimes that happen and lots of money being the reason for specific peoples actions and choices. While Drama Stage Season 3: blackout is a movie-like story and thus does not cover many topics Woman of 9.9 Billion does, their essence is still pretty much the same

School bullying. A girl that gets help from someone who is a dropout and kinda seen as the "bad one" and a sort of revenge towards the bullies. Both dramas are very well done and have a surprising ending. The theme itself is quite similar as well.

Both dramas are about married couples that deal with affairs. In both dramas the couples are going to no end to get their revenge. Both dramas are filled with the themes betrayal, vengeance, hurt, mental and physical assault.

Both are revolving around a group of police officers that work together to solve crime cases. In both dramas those cases have a special link to the officers. Both dramas deepen the cases by also focusing on the suspects and the victims. Both thematize cases that grip the viewer.
Cinematographic-wise and also music-wise both are also working with the same elements.