It's from The Necromancer, a part of the Witch's memories about Niten (The Swordsman/Miyamoto Musashi), narrated by Sophie.
I really like Benzaiten appearance, she is awesome with the pink dragon and how she tap its head with in a regal manner... The description itself, "tiny, ethereal, and beautiful" had made me speechless...
For her personality, I think she is a good Elder, if I judged from Niten, the Humani who she made Immortal. Usually Elders will give immortality to a potential Humani who has similiar personality with them. (Example : Kukulkan and Billy, who both have temper). If it's same with Benzaiten and Niten, then Benzaiten would be an Elder who valued the concept of Bushido, because Niten holds the Bushido.
Well, it's only an assumption. I will wait her actual appearance in The Warlock or The Enchantress.