Recommendations

Male lead falls in love with female lead in school age and is "devoted" to her since then.
Female lead plays an instrument.
Shy/reserved leads, slow burn romance.

They each deal with solving crimes and busting the bad guys. In both Signal and Lookout (Th Guardians) there are several main characters that work in a team and have very deep, heart-wrenching backstories and goes/focuses on each their individual stories equally in different segments of the drama. Sooji and Haeyoung have strong similarities regarding the crimes that changed and ruined their lives. The villains are especially vile and will make your blood boil in both, LOL.

While Lookout is not as superb as Signal (among my top faves!), I still strongly recommend it because it is good and have paces that will have you on the edge of your seat until the very end.

-Both female leads have similar personalities & life circumstances
-They involve family dynamics/children
-The mothers of the female leads are strong-spirited in both

They overall just have a similar feel to them!These are my top two dramas EVER & remind me of each other! Trust me and just watch them... ;)

It has a similar vibe. It depicts a same kind feeling of helplessness even if its roots are different. Their soundtracks are a work of art. Less than evil may focus its attention on the policeforce and its flaws while Cruel City shows the people living on the other side of the law.

Both movies portray a disaster in a city and struggling people to be survived.
Both movies benefit form great actors and scenario as well as eye catching special effects.

Both have a good feel vibe and will make you smile. Both won't make you cry and don't contain serious themes. Different stories,same warm feeling inside.

From the same director/screenwriter, both movies are historical comedies set in the Joseon era that center on a group of professional 'outlaw' experts in scheming and deceiving.

- Mystery set in a small town -
- Same eerie vibe -
- Wonderfully acted -
- Tension building OST -

watching switch reminds me of leverage, so if you want a series scamming bad people then you should watch both.

Both have good action choreography and both stories have the protagonist learning fighting skills to prepare for the final duel in a revenge subplot.

Same director so the same atmosphere of the Sol, several characters, several stories, both showing different views of the life in prison or the hospital, presenting the good aspects of people, both makes one have connections with the characters and got attached to them

technically a spinoff story with phun and noh from the previous seasons of ls, based around couples at school and in the same universe

Both series include a girl who supernaturally transforms into a guy and goes to live in a male environment. They both heavily idealise the idea of falling in love with a soul instead of a gender. The transformed girls in both series keep their identity as a female a secret for as long as possible.

Only different is the female lead in Duen Pradab Dao as a twin and was adopted by Americans, both Lakorns are similar, both female leads leave their foreign country such as in Duen Pradab Dao, the female lead lives in America and then she leaves America and pretends to be her twin sister just to get to know her biological family in Thailand, vs Ruk Pathiharn where the female lead leaves France after both her parents had passed away just to meet her family in Thailand.

Both movies include an lgbt couple with a kid, who have to face discrimination and prejudice. I guess the endings are also both left rather open.