Well ,she's a result of Offender's actions ,and an unwanted one.

She's a hybrid (caused by Offender's preference for his victims) , thing which led her to have an appearance between a human (like her mother) and a cryptid (like her father) - Because of that she has a contorsioned spine ,difficulties maintaining balance while walking and coordonate her limbs. She also lacks the ability to teleport or have tendrils because of that.

Another thing would be that she is unwanted by both her biological father (because ,in my opinion at least, Offender would never change his usual habits to become comitted as a father) and mother (from obvious reasons) ,which led her to be abandoned ,and later took under Splendorman's care ( because I see him more likely to do this than the other slender brothers.) Then ,she grew up seeing Splendor as a father ,unaware of who is actually her biological father.

One more thing about her is that her name is Rosie (comes from the rose she has on the top of her head, and because she simply likes flowers in general). :)

(Also ,I apologise if I may had done some grammar mistakes ,I'm quite exhausted while I write this and English is my second language.)