In both shows, the Male Lead is a Grim Reaper tasked with fetching souls. One of these souls they are supposed to fetch is the Female Leads'. But, in trying to accomplish their goal, they end up falling in love with her instead.

DAYS feels way more lighthearted than Black, though. While the former starts with some hilarious comedy at first, halfway through the series the crime element of the show turns the series dark (pun intended).

In both shows one of the leads gets close to the other for an ulterior motive.

In the case of Undercover, the ML is a secret agent tasked with infiltrated a student movement. So the ML gets close to the FL to get to the leader of the movement.

In Catch the Ghost, the FL joins the ML's team in order to conduct an unofficial investigation involving the disappearance of her twin sister.

In both shows the Male Lead pretends to be someone they are not in order to use the Female Lead. However, they end up falling in love with her and, in so doing, they betray their original mission in favor of keeping her safe.

Both shows feature undercover agents who were betrayed by their handlers and led astray. The Male Leads suffer from an identity crisis, wherein they have been playing a role for so long, they no longer know who they are.

The Female Lead in Undercover, however, has a bigger and more important role than the Female Lead in Cruel City.

Both shows feature a group of eccentric characters who fight for justice in their own unique way. The leads are both lawyers who end up working together against a common enemy.

Both shows:
> Happen mostly in a high school.
> Have highschooler as leads.
> The FL has a secret they are afraid will be revealed.
> The FL is popular in their school.
> The FL suffered at the hands of school bullies and still carry those scars.
> The ML is the smartest in the school.
> The ML is the silent but caring type who falls in love with the FL first.
> There are secondary couples in both shows.
> FL has a close group of friends.
> The ML has an estrangled relationship with his father.

The secondary couple of both shows have a similar dynamic, in the sense that there's a role reversal with the Female Character (FL's older sister in True Beauty & ML's older sister in Love With Flaws) being proactively persuing her crush. They are also both successful Female Characters who enjoy being in charge.

Both shows focus on protagonists who faced a lot of pain when they were children; including being experimented upon when they were in an orphanage. The Male Lead in both shows is morally ambiguous and hell-bent on making someone pay for what they did. They also have a soft spot for the Female Lead, with whom they share a childhood connection.

At first glance, these two shows would seem like they have nothing in common save for Kim Seon Ho being in both of them. And you'd be right... to an extent. But both of these shows also have:
> Kim Seon Ho plays a very similar character; loving, sacrificial, and incredibly nice. But while he's the 2ML in Start-Up, he's the ML in CTG.
> The ML falls in love with the FL first.
> One of the leads is not honest/keeps a secret from the other for more than half of the drama.
> There is a love triangle. In Start-Up the Love Triangle becomes the main plot; whereas in CGT there's a double love triangle but it's a small aspect of the plot.

They both are psychological thrillers with a small cast but great acting. Both movies also deal with a big betrayal and a big reveal as to the real personality of one of the characters. They also both kind of deal with "going back in time," though in a very different way.

The only similarities between these two are the writer's usage of "time-travel" or "time-altering ability" to further the plot. In FGTD the FL is caught in a time loop and the only way to break free of it is by keeping her boss from dying. In Kairos, the ML lives a month in the future and, by communicating with the Female Lead, they work together to help each other avoid disaster (which includes death).

FGTD is mostly a comedy with some melodrama towards the second half; while Kairos is a more serious drama.

In addition, the FLs in both shows are very active, opinionated, and stubborn; while the MLs are portrayed as aggressive and arrogant at first, but they change as they face problems, learn from their mistakes and begin to rely on the FLs.

Both shows feature:
> An orphan FL who must overcome a trauma.
> A skillful male lead who is shown to be a problem solver.
> One lead chases the other first; with the other being reluctant to accept their feelings because of a past relationship.
> One lead has a past relationship who ended up dying, leaving them with a sense of guilt they must overcome.
> Both shows happen mostly in a hospital.
> Different cases per episode.
> The MLs are eccentric.