Aria is Rea's stepmother. She used to be a maid for Dan'ichirō and was trained under a long-standing tradition in the Sanka family that future heirs are to marry the best of the maids. Dan'ichirō however met and married Rea's mother which he loved dearly until her death while giving birth to Rea. After the death of Rea's mother, Dan'ichirō succumbed to deep depression and soon deprived himself of further nourishment, which Aria then slowly nursed him back to health.
Although Dan'ichirō married Aria afterwards (to her joy), he does not give her real attention and instead watched over Rea doggedly, which has made their relationship a strained one at best, to the point of having no interest at sexual intercourse whatsoever. She was left out of the Sanka family, and watched over the distance as Dan'ichirō spoiled and restricted Rea. Because of this. Aria secretly despises Rea and how she is loved by almost everyone. Later on, after confessing everything to Chihiro, she tried to seduce him but ultimately got rejected.
Aria is a miserable and depressed woman. She is also apparently alcoholic and drinks a lot, perhaps due to her unfortunate life. Aria runs for her husband's affection but he could not care less. She is very resentful of her step daughter because of her husband's near incessive obsession with her. She greatly laments not being wanted by anyone, especially her own husband. She may make the accused attempt to get her husband's attention but has largly accepted his indifference. She feels sorry for herself and tries to get others to do so, as shown with Chihiro when she even tried to seduce him largely by insulting Rea and telling him it's been so long since anyone had touched her. When he rejects her she begins to sob hysterically asking why won't any one pay attention to her.
It's shown, however that she wasn't always so bad. In the early years of her marriage she was caring and soft spoken when Dan was nearly depressed after the death of his first wife. She then nursed him back to health and was very happy to marry him. She was kind of enough to think her that husband married her out of love despite all the evidence to the contrary. Initially, she did not resent Rea and felt sad that she wasn't really included in the stuff she and Dan did together. And when they went on vacation Aria asked if she would could take a picture with her which she accepted implying they may have initially had some similarities of a normal mother-daughter relationship. When she found (naked)pictures of Rea that her husband took, her jealousy of Rea only grew.
Its implied she may have a soft spot for animals as when Bābu cuddles up against her, she gets excited and ask him if he wants to come live with her and be her friend though Chihiro did catch her at a particularly vulnerable moment and she quickly cast him away when she learned he's a zombie.
she does have a good side where she doesn't stop Rea from occassionlly sneaking away from the manson, in fact she does wish that she be freed from her controling father.has of episode 14 after having a wierd shard dream. with rea and furuya. she is finally starting to let go of her resentment of rea.