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They both are crime and suspense dramas. The leads have amnesia. They have amnesia due an accident during a mission. Oh and of course they both have a little romance in them.

They are both crime and suspense dramas. They both have one case that leads the next and ends up being an entire conspiracy. I mean they both have gorgeous leads and a little romance in them.

Pretty similar! A bunch of young kids with some kind of supernatural power! It felt very much alike to me :)

Both deal with a male lead trying to get close to the female lead although her memory resets after a set time. In "One Week Friends" the time period is a week, in "Okitegami ...", the lead's memory resets every time she wakes up. Plus, in Okitegami she's a detective.

It gives the same kind of vibe... probably because both dramas have the same main actress but also because in both dramas they actually shouldn't be a couple because of their huge "differences". Watching Legend of The Blue Sea really reminds me of the time when I watched My Love From Another Star.

In which the main character unintentionally forgets about the past.

I recommend Sunny if you really enjoyed Age of Youth! Just like Sunny captured a snapshot of young adulthood in the 80's, Age of youth also shows the complexity of being a young adult in the 21st century. They both involve love, friendship, family, and death. There are also a lot of similarities between the characters in both, and if you enjoyed Age of youth you will enjoy Sunny.

both stories center around two high school girls who don't do well in school and have bleak futures, and their tutors who motivate them and help them get into prestigious universities

A family drama with just as many chaotic relationships. While Father, I'll take care of you revolves around one main family dispute and how that spreads throughout the family, King's Family has a lot more going on around every character's relationship and problems right from the start.

Both main male character avoids his girlfriend so she won't be sad when he dies. Both are very sad and emotional.

Both:
Are fantasy
Are sci-fi
They involve the past and the future
Same actress that playes the main role
Same atmosphere
Sweet romance
Funny moments
Sad parts
Bittersweet life lessons

They have the same actress playing the lead couple and the couple in both dramas are so sweet and have adorable chemistry. They're both eye candy as well.

The chemistry in both dramas between the leads are sweet/adorable and made me root for them, though Love O2O has less obstacles.