It was sometime during Season 6, probably closer to "Every Little Thing She Does", that I really grew attached to her.
I remember being super autistic and concerned about how she'd affect the show in-between Seasons 5 & 6, even asking one of the staff members about it at a con at how exactly they were gonna handle adding a new character and what it would change.
I think what probably sealed it was "To Where And Back Again". The way she took charge, the way she faced certain fucking doom numerous times in the episode, etc etc stuck in my head along with the development she'd gone through throughout S5/S6 and it really endeared me to her character.
Then everything from S7-onward was just fucking adorkable and solidified her as Best Horse for me.
So yeah, that's my Glimmer Autism story