Very Brave and devoted Male lead
they are similar in styling , vibes and era I think
One and Only is more to the romantic side but Nirvana is more to the political side
non the less ,,,, I feel their similarities watching it

Friends and family are everything to the young
the dramas are similar in the comedic tone and the friendship
also the family values

Same Miserable couple
Same Youth problems and defying all obstacles
Same painful hardships ,, different details

They are the same school romance drama ,, the second male lead has psychological problems in who are you school 2015 drama
the same in spring is green the first male lead has psychological problems because of his parents
Youths problems and romance

Both republican era dramas
Both have struggles in desperate times
Both are fighting to protect their country and keeping it safe in chaos and war
killing people is the most common theme and it shows How turbulent time it was

Both Youth dramas with tournaments and the journey of achieving goals
Winning is not the goal but having good friends that back you up every step of the way is the most important message in both of them
Both have amazing Values to every young man and woman in competitive field no matter what the kind of the sport is ,, it's the same

Similar Sports drama with a lot of friendship dynamics and family like in sports spirit
Both are really good youth dramas with positive messages

The same Republican Era
The same war fare and spy work
The same antagonist with miserable love life and revenge
even the decor and styling are the same

Both of them are Republican era dramas ,, Both have amazing Bromance or Brotherhood relationship ,, Both have the same painful stories of this era fighting the corruption ,, a lot of killing and action scenes in both dramas
Both you will feel how strong relationships can be formed without having it to be blood related or romantic aspects
I loved Both dramas so much

A cop from the 80s goes into a tunnel comes out thirty years later to solve an old case
Quite similar synopsis for both of them
Retro detective didn't air yet but from the synopsis they are similar and Tunnel the korean drama was amazing one ,, you should gave it a shot

Similar of the ability to read minds also the male leads falls ill every time he reads people
,, it's non romantic though

similar bromance story with great chemistry between the two male leads
also both in republican era<<<<