Female lead is a die hard fan of someone. Femal lead has a dirty mouth (oohh~)

1997 - Siwon; 1994 - Najung

Female lead lost a sibling

TT_TT 

It's too sad.

Too sad.

Female lead has a childhood friend. He's a lazy but is incredibly smart.

1997 - Yoonjae; 1994 - Trash (sseuregi)

The shy guy is close friends with the loud mouth

1997 - Hakchan x Sungjae; 1994 - Samcheonpo x Haetae (Suncheon)

The only other female character who is close friends with the female lead and is a huge fan of an artist

1997 - Yoojung; 1994 - Yoonjin

The acting idol is a sweet guy that ends up liking/is fond of the Female lead's childhood friend

1997 - INFINITE's Hoya as Junhee; 1994 - B1A4's Baro as Binggrae

The random relative (of a different character) arrives and he likes the female lead and causes a freakin' love triangle!!!!

.

.

.

Saving the best and most obvious for last

Sung Dongil and Lee Ilhwa are still the parents in both seasons.

.

.

.

CONGRATULATIONS. You just finished this blog post. Now, go watch Reply 1994 before TvN removes it from hosting sites.