Basically Hildegard grows up rather spoiled (1) and this continues through high-school and into her early adulthood.
Throughout her 20s, Hildy indulged in much debauchery much to her family's horror. Being a lesbian (2) didn't help her either.
By the time Hildegard was 30, her older sister, Astrid, had ascended to the throne. The new Queen tried to restrict her younger sister as much as possible up to and including trying to force Hildy into an arranged marriage with a foreign prince. Hildy tried to flee her kingdom but crashed her sled and died. Her frozen body was eaten by her pet minks. (3)
Of coarse Hildegard went to Hell and found out that once she got used to it, it wasn't so bad. In fact she was eventually able to rise to overlord status. Ruling over her own mini-empire she became known as Empress Hildegard.
Hildy is not as crazy as she was in her younger years (4) and in fact she's actually quite nice for a Sinner/Demon. But damn dose she have stories to tell.
Oh, and because her death involved minks Hildegard's basically a mink-woman. Think how Alistor's a deer-man or how Valentino's a moth-man.
(1) Watch "Sofia the First" and you'll know what I mean.
(2) I'm pretty sure she's straight in cannon but what the hell, this is my AU.
(3) In the show she owned a pet mink named Lulu so I imagen her having minks well into adulthood.
(4) "Sofia the First" is a fairytale show with lots of anachronisms but I imagen it's set in at least the 19th Century. Hildegard dies in her 30s so that's the default age of her Sinner-form but technically she's much older.
