I think I played too much Triple Triad xD
I went back again at the Library and she was there in front of the counter, I don't know why I was expecting her either behind the counter or near a book shelf, and I was also looking to her name in the dialogue box, and her text for this scene is a dialogue between her and another girl, and for once there is no mention of her name "Pig-tailed Girl" in the dialogue box, so I don't know why I thought that girl wasn't her, I think I focused more on challenging everyone to Triple Triad...
Anyway, my bad, but at least this allowed me to discover Hyne more deeply and above all to try Deling, a great tool to avoid some laborious video compositing to create parody, etc.
Thanks for your time, your tools and help myst6re.