Ryoshu was created by the House of Spiders for the sake of wielding Arayashiki, a Relic in the form of a sword that erases whatever it cleaves through as though it had never existed. However, wielding this power takes a toll; Ryoshu's own memories are shredded whenever she unsheathes the blade. She is canonically shown to dissociate and struggle to recall her fragmented memories, and due to the abuse she suffered at the hands of the Nursefathers, the figures in her life who were supposed to be her parents, she very likely has the trauma.
Seele Vollerei herself is a modest, sweet, kind, and caring girl, although being rather meek, as she was unable to stand up for herself or for Bronya when both were being bullied by Sin Mal. She also appears to dislike bugs.
With her insecurity growing, a flower bloomed within Seele; the waking of the Stigma's personality, Veliona. Veliona was like a polar opposite to Seele, being cruel, ruthless and sadistic. Veliona, however, appeared to be affected by Seele's resolve to save and care for Bronya, perhaps even coming to care for Bronya as well.
As Seele grows stronger and desires the power to save Bronya, Veliona hears out Seele's request for help and the two come hand in hand as dual-egos.