Istina has a very single-track mind. Outside of the battlefield, she only cares about books, and on the battlefield, she only cares about using those books to kill people. Your talent, Researcher, gives her an increased ASPD in exchange for DEF, which allows her to fire her slow spells at enemies with great consequence. Usually, however, she can only slow one target at a time, which is why she uses it Literature storm as her S2: an ability that boosts her ATK and allows her to fire multiple magical bolts simultaneously over an increased attack range, thereby inflicting Slow on all of her victims. I mean – actually no, “sacrifice” sounds about right.
Istina is quite strong, but she has big shoes to fill compared to her 6-star superiors Angelina and Suzuran. When Angelina was the only 6-star backer in town, Istina had a pretty serious niche – Angelina can be tricky to use, while Istina is reassuringly simple and effective. Once Suzuran comes out, she takes Istina’s skills to the next level, which leaves Istina in the cold.
That being said, if you don’t to have Angelina or Suzuran, Istina is still an excellent choice. Boosted ASPD gives her pretty good DPS and despite her low base ATK Literature storm is extremely efficient, even if the uptime is not the best.
I recommend moving for Istina if you don’t have Angelina or Suzuran. If you have both, it probably isn’t worth pulling on directly, but it can still be worth increasing them to get additional art damage and crowd control during the challenge stages.