Destruction (2021)

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).

Here the main lead character of the drama is the doom itself. He soon falls in love with Tak Dong Kyung, who has just 100 days left to live. The male lead helps fl in both!!

Seo in guk is male lead in both.
ML is an antihero in both these drama.
Character of ML in both dramas have a melancholic vibe other than when they’re with FL. it’s only when ML is with FL that his personality becomes quirky and chirpy.
Both have FL who lost parents at a young age

Although different in tone (DAYS is darker), both are about a human FL who makes a deal with/becomes involved with the ML who is a personification of something (math vs. doom). Both MLs are cold and distant at first but become sweet when they fall in love. The FLs end up living at/attached to the ML's house which is sort of cold and barren, reflecting their life. Both dramas have a scene of wishing on meteors.

Both female leads find themselves in a difficult situation until they meet a man that will supposedly save them. They find themselves in situations where they end up faking their love (DAYS - so that he will die) (CLOY - So that she doesn't die or get caught as a South Korean). Through this, they both bicker quite a bit but eventually gain feelings for each other. Although they start to like each other, they both do not realise. The male leads have both made the promise to keep her safe but when she sacrifices herself, they become quite frustrated and realise they may have feelings.

The FL gets diagnosed with cancer and has given numerous days left to live. Because of this, she decides to quit her job and live her life to the fullest.

The story is similar to Doom at Your Service. You can feel a connection between both FL’s. The only crucial aspect that is different is that it doesn’t have fantasy.

In both dramas, the FL has a younger brother whom she cares for deeply. Both FLs have a brush with death.

Even though at first glance they don't seem to have anything in common it's the atmosphere and the overall feel of the romance. In both shows the affections and bickerings between ML and FL are similarly adorable, feverish, loud, heartwarming, sad, joyous, melodramatic, hurtful, amusing, ...
They take you for an emotional ride and you become very invested in the relationships. Also in those of the supporting casts.

In both series, ML is a supernatural being that is kind of struggling with his existence. Both end up falling in love with FL, who is different from everyone else (in MMIC FL is basically cursed in love because of her past life; in DAYS FL has terminal cancer). And in both series ML disappears for a while but returns as a human.

even though the elements of the worlds are different, the dramas are very similar:

• they’re both fantasy.
• in both, the FL, at first, has a trashy boyfriend who leaves her for another woman.
• in both, the MLs are so precious and care about the FL as if it’s something they can’t control. <3
• they both have beautiful family and friend relationships.
• in both, FL is given a special chance by fate (in DAYS, she makes deal w ML to live cancer-pain-free until her death and one extra wish as long as she wishes for the world’s doom before her death; in SFOT, she travels by deleting old msgs to parallel universe in which she’s still in highschool and can change her failure life as long as she makes sure the ML doesn’t die).
• in both, there’s the concept of “no matter how many times fate pulls us apart, i will find you and love you all over again.”
• in both, the ML will do ANYTHING to save the FL even if he has to sacrifice himself.
• in both, the FL tries to change their ill-fate but fate wins every time.
• they both have side characters’ love story which doesn’t change in this game of fate.
• in both, there is a beautiful twist of fate in the end.

but they’re also different from each other:

• the love story of DAYS is enemies to lovers and is set in modern day while in SFOT it is strangers to friends to lovers (but the ML has always loved the FL) and it happens in 2010.
• DAYS couple are 30-ish yrs old (although ML is immortal, he looks like a 30 yr old) while SFOT couple are in highschool (FL is mentally 30 and travels back in time).
• in DAYS, the ML is not human and a deity exists in whose hands fate lies while in SFOT everyone is human and the rules of universe control fate.