A tragic event in Seele's past made all timidity be wiped from her forever, until she returned like a mysterious butterfly. After Seele's return she came with newfound scythe-wielding skills from deep within... Veliona is the name of Seele Vollerei's alternate personality sharing the same body, and mind but with drastically different personalities.
While Seele comes off as a very shy timid girl who wholeheartedly loves her fellow orphanage family Bronya Zaychik, Liliya Olenyeva, and Rozaliya Olenyeva the same cannot be said about her other darker side Veliona. As Veliona has a blood-thirsty, callous, and sadistic mentality where she sees no issue with abandoning that which Seele holds most dear if it's to greatly benefit her hosts's body itself or if it's to appease her own enjoyment in violence.
Veliona shows up in-game under Seele's Swallowtail Phantasm valkyrie form during her ultimate bursts wherein Veliona takes over bodily control with a noticeable change in her atmosphere with her eye colour changing from blue to red swinging around the scythe in audible glee while maniacally laughing the entire time.
However when Seele is deployed under her Stygian Nymph form they exchange places entirely with Seele's original white dress with a flower in her hair changing over to a much more sinister black dress with a third eye appearing in place instead.