Y'shtola is a Mary Sue.

[Zabuza]

More mature, more assertive. Earlier she was like a youngster pretending to be a big girl. After some experience her words and actions carry some weight. There's not as much difference as Alisaie but Shtola was a little further along when we meet her.

Assertive as opposed to what? When was she ever... not assertive? Mature? She rejected a certain Au'ra's advances in the most savage way possible(The Burn Savage). A more mature lady would have rejected him in a more compassionate manner, even if he did deserve to be knocked down a peg.

Way to forget about the rest of the previous Scions.

Poor Minfilla, Papy, and Lyse are betrayed yet again by the community.

Also, replay Stormblood. Y'shtola shows more emotions there such as her desperation to save Lyse and realizing that Zenos is far beyond even the WoL early on.

Lyse had a whole expansion devoted to her. She's a polarizing character that people either like or dislike. She's like the complete opposite of Y'shtola. She's impulsive, emotional, and has been shown to make mistakes frequently.
Unfortunately, Papy was kind of thrown away for Lyse's development. Minfilia was a pretty boring character as well who I feel emotionally and mentally remained the same throughout, but I don't have to deal with her anymore.
Lyse, Papy, and Minfilia are either largely out of the story or out altogether.

Y'shtola has shown the barest hint of having emotions and only in the most extreme situations. Sorry but I don't consider 'Lyse, Run!' or 'Such infernal strength!' some kind of emotional epiphany.