Not much is known about her, except that she shows obvious favoritism in Gart, as displayed in her debut episode.
As mentioned by Gart in "Between Brothers," the episode, "Grandma's Day Out" confirmed his statement of being the favorite of the Default siblings when Robot blamed him for placing his responsibilities on him and Monster to care for their grandmother, resulting in her being taken by a Howling Cybermonkey in said episode. At first, Arpa appeared furious about having seen it all on the Tube, however, she stated how happy she was of what Gart had done because her mother said she had fun despite all the danger she endured. She embraced Gart while scolding Robot for "tattling," overwhelming her younger son with frustration as well as painful feelings of being a disappointment in his mother's eyes once more.
She doesn't keep discreet when it comes to letting Robot know what a disappointment he is to her and the Default family name, and Robot usually takes it in stride, having become immune to her harsh statements. Although Robot usually mentions his father in most episodes, it is evident just how much of a negative impact his mother had during his childhood as well. Much like Gart, as well as most of the Default family, she is quite discouraging of Robot's inventions.
She made her second appearance in "The Party" where she, Gart, and especially Grandma were horrified when the secret slipped to Robot that Gart was going to host a party in honor of his portrait unveiling. Reluctant that Robot now knew about the party meaning he now could go, Arpa threatened to "cut him off from the family" if he embarrassed them, especially in front of high-society personage, Lady Madame.
She shows to have great dislike for Crazy Cousin Gizmo as well, as seen in "The Party," as she had tried to have him thrown out of the family party while Robot made every attempt to cease it. Without a second thought, she had decided to have both Robot and his cousin detained, leading to the destruction of her older son's party.
Her judgment of those unlike her and every sophisticated individual is greatly negative, as she is suspected to have strong dislike for Monster. As displayed in "The Party," contrasting with the usual events of distinction that the Default family hosts, "fun" is strictly scorned and those who seek it were violently thrown out of the party.
However, despite her cold demeanor and her open dislike for Robot, it was revealed in "Family Business" that she secretly does seem to love him, her feelings portrayed as a "show, don't tell" action. She appeared to be unaware of her or her family's own hurtful judgment of Robot as Monster pointed out, and even agreed to allow him to teach them how to appreciate Robot more (although, this was mostly to win Robot back from Pendulum Depot's manipulation to steal the blinking light formula). Though their actions toward the end of the episode were proven poorly as Robot stated they only pretended to like him so he wouldn't hand over their formula, Arpa, as well as Gart and Grandma, were pained when they assumed Robot would actually turn his back on them (when he actually tricked Pendulum Depot with a formula for exploding lights).
In said episode, it is revealed that she keeps a picture, which was torn from a portrait, of Robot inside her hatch, literally "close to her heart," which Robot discovered. Toward the end of the episode, out of Robot's earshot, Gart questioned why exactly she keeps it and her response was to "keep her loose hatch in place." However, when Gart grew excited and was determined to let Robot know the new discovery, Arpa shushed him about it. Judging by the way she spoke as well as her expression, it's confirmed that she may have love for Robot after all, although she would like to keep that secret, even from her oldest son.
Her appearance is very similar to that of Gart, confirming that he inherited his genes from her. However, she is much taller than him and her arms display a darker orange gold color and appear much bulkier. Much like her sons, she displays the first letter of her name on her chest. She has circular disks that protrude at the sides of her head, similar to hair buns. A noticeable feature about her appearance is that, unlike Robot and Gart, her eyes are much smaller and she only has two wheels. She wears a long pearl necklace to display her wealthy status.
She might possibly have inherited her appearance from her father as she looks nothing like her mother.
- Her pearl necklace is almost similar to Nessie's, though it is much longer.
- Like Gart and Spitfire, she has fingers.
- She is the second relative from Robot's family to be seen.
- Her eyes resemble petite glasses.
- Her personality seems similar to Arianna from The Cleveland Show.
- It's clearly unknown how Arpa and her husband and other robots can have babies, since they don't have organs, but it could be the same way as the robots from Futurama, when the creators decided to make an episode of how robots can have babies in "The Bots and The Bees."
- Her name is Italian for "harp."
- She may care for Robot, as seen in "Family Business," she holds a picture of a young Robot in her chest.
- Her body mold strongly resembles that of Gart, though it is possible she may have inherited it from Robot's grandfather as she looks nothing like Grandma.
- She is the fourth female Mechanical to be introduced. The first was Spitfire, the second was Lucy, and the third was Grandma, who appears earlier in the episode, "Grandma's Day Out."