The woman warrior
The woman warrior is such an important character, because we often forget that women are just as able and called to adventure as boys are, because typically, the hero is depicted as a man. Luckily for the reader, this story is about a true hero, and Aerin shows what the hero's journey is like when a woman undertakes the challenges. Long story short, she kicks butt and establishes herself as an authority.
The male divine helper
Arthur saw a blonde woman with a sword in a lake, but Aerin sees a man, a man she quite likes actually, and she even promises to come back one day. But he is her divine helper, giving her the sword she needs for her quest, and telling her of her mother, just as the divine helper in a man's story tells him of his father.
The "Hero's Crown"
If it's a "hero's" crown, why does Agsded have it? Perhaps it's named after the fact that it has to be won, but it's important to notice that when Aerin wins it, she passes it along. Perhaps she knows that she is too complex to be a simple "hero," something that Tor seems more fitted for. Instead of claiming her rewards in battle, she often gives the relics away to enable those she trusts.
Aerin's royal magic
Instead of finding that she doesn't have royal blood, Aerin learns that she has royal blood and the blood of her mother, a powerful witch. This makes her exceptionally able to slay Agsded, a task no one else can. This is ironic, because the novel begins with the idea that maybe Aerin doesn't have royal magic after all.
The marriage of Tor and Aerin
Aerin promised to return to Luthe, but marries Tor instead, after battling alongside him, remembering his friendship and humanity, and knowing what would be best for the kingdom. And yet, she chooses the healthy, responsible choice instead of following her fascination with Luthe and his dark magic. This seems to represent a dual nature in Aerin, but one with the discipline to do what is right instead of following indulgence and becoming like Agsded.