Kafka is obvious fanservice, and was clearly used to advertise to male fans in the build up to game launch, so her popularity is understandable.

Blade's popular probably for the same reason Draco is.

Jing Yuan is popular for representing Space!China. But is too goody two-shoes as a character to enjoy much fic following.

Aventurine definitely seems to be the dark horse of this race. His popularity came out of nowhere. I guess it is because Penacony shoved all of his sob story into the main questline instead of a personal quest line.

Looking at the comments, I like this person's suggested timeline:

藤宵战死余倏忽之乱，景元前往元帅之处授封，丹恒应星造就孽龙，镜流带队诛杀孽龙，景元授封回来刚刚碰上引月之乱，这个顺序理论上是合理的。