Why are so many people surprised?
If you haven't understood BM's character until now, I can't help you.
She's heavily inspired by the Majin Boo character of DBZ.
She has a innocent, child-like side (aka "Good Boo") and a dark side (aka "Evil Boo"). Those two sides are also represented by Zeus and Prometheus.
Her dark side probably has something to do with Carmel and the Soul-Soul-Fruit, since we've seen her being a goodhearted child back in the day.
I'm pretty sure the endgame with her is that somehow her dark side gets seperated from the original O-Lin character.