Actually some of us just dislike the character for being a generic supersue.
I don't see her that way. Fiona is an elven mage who came from an abusive home under an Orlesian lord who treated her as his personal pet, and lived a horrible life in the Chantry controlled Circle. She earned Duncan's respect as well. According to the codex on the axe named after her, "When Polara disappeared—perhaps recalled to Weisshaupt, although none in Ferelden saw her leave, nor have seen her since—Duncan bequeathed to her son a silverite axe of great value. Duncan said he had wielded it and a twin in younger days, back when he had been a far different man. He'd named the axe 'Fiona' after a Warden who inspired him, and suggested that the son ought to take similar inspiration from his mother."
Despite everything she's been through, she forsake her own freedom to return to the Circle, and help her people: “I came to the Circle from the Grey Wardens because I saw something had to be done. In the Wardens, we learn to watch for our moment and seize it - and that moment is now.”