Spoilers for Fate/Grand Order!
In one of the events, the girl on the left, Jeanne Alter, created a bunch of fake versions of different characters. The long-haired girl, Brynhildr, was one of the fake copies. She ended up being quite different from her true version, and was really, really gay for Jeanne Alter. Notably, JAlter didn't actually mean for her to end up that way, and only intended for the fake Brynhildr to be her friend.
During the event, the fake Brynhildr fades away, along with the other fake characters - being copies, they had a limited lifespan.
Where this comic picks up is some time after the event, where Jeanne Alter comes across the real Brynhildr. Unlike her fake version, Brynhildr is more quiet and withdrawn. Upon seeing her, Jeanne Alter is reminded of the now gone fake Brynhildr, and how - despite being supremely annoyed at how clingy she was - Jeanne Alter actually misses her. Because of that, Jeanne Alter wants to try and get close to the real Brynhildr, in memory of the fake one.
Hope that helps clear this up! Also I agree that this comic is absolutely adorable. And a bit sad as well.