The only thing i'm going to critique is her passive being basically just like kennens but with a 3 second delay between stuns. You might try finding a different passive for her since it's litterally almost exactly the same as kennens.
"Enemies hit by Kennen's abilities receive a Mark of the Storm for 6 seconds, stacking up to 3 times. At three stacks, the marks consume themselves to Stun the target for 1.25 seconds" the only difference is his can be applied as many times as he wants with diminishing effects.
So if i can make a suggestion, changing her passive to something more fitting of the name "Lost Chapter" maybe it empower's her other abilities, maybe move her ultimate to her passive somehow and then give her a different ability for her ultimate (but with the same name as it currently is) cause if we think about it, imagine she's reading her book and that's what let's her use abilities a few more times in a row with no cooldown. Just a suggestion, litterally not telling you that's what i think her passive should be just trying to help get your wheels turning.
Otherwise this is pretty amazing and great job.
Edit: No lie i love the color scheme, those are all my favorite color's, the abilities aside from the passive are really neat and unique and i'm sure she'd have a place in league of legends if her passive was different.
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