Tsubaki's past is unknown to us, but that doesn't mean we can't make our own rendition of it. This is how I imagine Tsubaki's time with C3.

A young girl now orphaned after losing her parents in a freak car accident. She ran away from the orphanage to find herself asleep in a church. Seeing two young boys watching her. What happens next find out. p.s. doesn't get to modern time till chapter 14. bear with me please

Might not update a lot but oh well