Allyria is around the same age as Arianne.
(Dornishwomen tend to marry later than women in other parts of Westeros, and her marriage to Beric was actually going to happen within the next few months when war broke out.)
Basically idolizes Ashara's memory, and they were as close as they could be before she died.
Actually looks a lot like Ashara, though with some differences (Allyria has brown eyes instead of purple, things like that.)
Does not like to think of Arthur. At all. As far as she's concerned, he's a traitor to their family and nation.
She was very close to her other older brother Allem, however.
Go look up the trope Lady of War, and that should give you a decent view of her character.
Badly wants to try claiming Dawn and be the next Sword of the Morning. She hasn't yet because she's held back by the knowledge of what Arthur did and the fact that she's supposed to marry outside of Dorne.
Genuinely likes Beric Dondarrion, and truly thinks that they could end up loving each other.
However, she's not exactly enthusiastic about marrying outside of Dorne- especially to a Marcher lord. She's never quite forgiven Allem for pushing the match even if she agreed to it.
Remembers enough about the Rebellion that she knows that Ashara never had a child. She's chosen not to think about it, and told Edric that they might have a nephew/cousin in the North to let him think they're not the only Daynes left.
Doesn't have as sharp of a tongue as Ashara did, but she has her moments. Mostly around their cousin Gerold, who hates visiting Starfall for this exact reason.
Loves embroidery and weaving.