Title of an anime with a certain side-story
Finally! I remembered what it was! 

The title of the anime I was searching for was Myself; Yourself.

The young girl was Hinako Mochida. The guy he liked was Shusuke Wakatsuki, who was one of Sana's (male protagonist) childhood friend. And she ended up with Asami's cousin. 


  Hinako is ten years old at first but becomes eleven in the course of
  the story. She is in love with Shusuke from when he saved her from
  three bullies who took her bag and would not give it back. She seems
  to want to learn how to be the girl he will like and asks Sana and Aoi
  for help. Hinako has a personality comparable to Aoi's, however being
  only in the fifth grade, she is childish as a result of her age. In
  the last anime episode, it is shown that she is the girlfriend of
  Asami's cousin, who is five years younger than her.


This is where I checked if what a friend of mine suggested (that the anime I was looking for was Myself; Yourself) was right, and he is.