Err.... This seems like more of a character study with the linkages between charas being the driving force so while I'm hesitant to plan things out super thoroughly, here are my thoughts for the overarching interaction between characters.

[@ScreenAcne] and [@Jove] are obviously linked by this secret society of the Dead Witch. They would play characters that are in the know about the Witch and the mis-happenings around town. Motivations are pretty fluid but maybe they are a group that wants to protect the town by guarding the Witch Tomb or maybe they want to revive the Witch? It is up to you two.

[@Mae] and [@Cao the Exiled], I like the idea that you two stole this artifact that was keeping the Dead Witch imprisoned. It lends itself well if the society wants to protect the town and prevent the resurrection. However, if the secret society (or even Mae and Cao) wanted to, their motivations could be the resurrect the Witch and the need for the artifact becomes even more important. Speaking of...

My character could be the ideal vessel for the Witch to posses or something? I dunno specifics but this is the first thing I thought of. Maybe Cao wants to get revenge at my character and sacrifice her for his own ends and maybe the society knows my character is important through lineage or something so I'm given some Weird Science thing for protection by Screen. Or maybe Cao is knows a bit about the secret society and knows Jove and/or Screen are in the society and want to resurrect the Witch so my chara somehow gets roped into the "Get Even" bit with him. Especially if my character was given some Weird Science Object that caused bad things to happen to her...

It really depends on what motivations you want your charas to be at!