SPOILER
There is another sidequests which continues off the assasin which is easily missable. Once you're back a mailfairy will deliver a faulty bomb. Bring it to julie your local dealer and she'll direct you towards scorch the girl that sits around the well. Scorch will speak of the harpy merchant. The harpy merchant in turn will lead you into a cave to retrieve a diary page. Lastly the diary page belongs to the green haired girl outside the cafeteria during daytime. She'll lead you to an abandoned storage cave. A strong Yvette boss will fight you. Defeating her will drop a serpentine core. Additionally the storage cave has a ladder in the top right. It leads to a tower in which the entrance is sealed. I've been unable to use any items to open it.