Fiona is one of the most powerful minds to have been born to Oberon. Unfortunately, her father (the King of Amber, and therefore the trend-setter for everything she knew) was extremely misogynist, and didn’t think females had worth, or abilities, beyond breeding. Having all of her succeses, and achievements either recognised as the work of one of her male siblings, or belittled, and tossed aside as unimportant, she learned that to get anywhere, she had to have a male puppet, or do it secretly, and convince other people that “it just happened that way” or the like. Brand was so often her manipulated toy, it’s possible that’s why his mind broke the way it did. Her attunements to various powers, her skill at forging, her playing her brothers against each other, all of her secrets, plots, and intrigues are all about a very intelligent, and gifted mind, trying to justify her existance to the one person who mattered, the King. Even when she thought to please him, by breeding, he still was dismissive of her, since her child Rhiannon was a girl as well.

Various Fionas in various Ambers have each created a Mjolnir which is a psyche-to-warfare transmuter. This tool enabled her great psyche beat some of the greatest warriors of all times. It still wasn’t good enough for her father to consider her worthwhile.

Her end wasn’t even remarkable, being that her mother, Clarissa, imprisioned her in a Grey Sphere, and then used up all her power/life force.