The AA from AA72 stands for Amusement Android. Unlike the standard MK series, this new android is specialized for entertainment and games.
The new AA android series was designed to feel a sense of accomplishment after performing their specialized functions and duties. This was done to improve the common personality traits androids have of not wanting to work. In AA72's case, she's loaded with a fireworks feature to decorate the summer vacation's finale to entertain the user.
AA72 lost her will to live from not receiving any orders until summer arrived. Since AA72 was specialized for summer vacations, this was obvious. However, AA72 didn't get a proper hand-off of duties during her release. She mistook her absence of orders for her being useless. AA72 considered herself as trash and headed to the trash can at the beach near Heavenhold. That's where she met Lifeguard Yuze. AA72 figured out her misunderstanding and learned about herself. Along with her newly formed memories, she also earned the name Stella. AA72 took a liking to her new name, and always adds her custom name Stella whenever she introduces herself.
Due to her trauma caused by concerns about being disposed of, AA72 gets anxious if she doesn't get any orders. When she doesn't get any orders, AA72 approaches her user with a hopeful look of receiving some while appealing her features. The other androids to not understand her. They feel envy towards AA62 getting to rest year-round after the launching fireworks once. Mk.99, who's particularly busy from battling, recently got caught changing her label to AA.99.
AA72 constantly searches for work to prove that she's needed. She usually helps Yuze in protecting the beach or runs errands for Loraine. If it's helping others, she usually accepts even if the request is slightly overbearing. She even accepts Sohee and Marianne's request of being a subject for experiments. Thanks to Heavenhold's residents' reports and Loraine's vigilant watch, the experiments were stopped. However, after the incident, Loraine became tired and halted all production of the AA series, claiming the Mk series is better.