In the anime, she isn't aware of the fact that he was the Masamune who she used to hang out with and used to love in her childhood. She seems to not even remember rejecting his confession and giving him nickname 'Piggy' and is neither aware that she is the revenge target Masamune has in which his plan is to make her reflect her actions she has done to him or others until now. He wants to make her fall in love with him and reject her brutally as she has done to everyone.
She first met him by accident when he was running away from some bullies and when he realized he was inside Adagaki property. From that point, they are shown to have been hanging out together in her garden. It is shown that he was in love with her and the feelings were mutual as she was also in love with him. When he confesses his love, Yoshino pretended to look like Aki rejected his confession and gave him the nickname 'piggy'. This is the main reason which triggered Masamune to train his body and become thin and handsome so he can take his revenge on her.
CHAPTER-27 (The Picture)
In high school, she starts to pay attention to Masamune after he finds out that she is a glutton when it comes to food which was always kept a secret. She soon gets hungry as soon as she hears anything about food. At starting she just annoys him or pesters him. But it all changed when he protected her from a jealous boy who she rejected and made fun of in cost of getting hurt himself by the scissors up to the point of bleeding which he tries to act cool and make it look like it doesn't hurt much which actually hurt him to the extent that he wanted to fall down. Later on, they gradually become close and she falls in love with him. Even if it were obvious, she denies her feelings for Masamune when Yoshino confronts her about it, being a tsundere and all.
In Chapter 39, when they learn of Kanetsugu being an impostor, she states that she wouldn't mind dating Masamune after learning he was the real one. Although embarrassed, she kept her stubborn demeanor. After dating a bit, she invited Masamune on their "last date," before she decided to break up with him. In chapter 48, she sends Yoshino to confess her love for Masamune, and that her true happiness is for her to decide. The last chapter shows that Masamune has made up his mind and that he wanted to give dating with Aki another chance, having rejected Kowai's confession. Masamune is convinced repairing their relationship will not work, but Aki blushed and says "It's not completely hopeless." She finally asked Makabe to kiss her properly, ending the series with Aki and Makabe together.
In the after school arc, Makabe and Aki seems to date each other secretly without telling others except Yoshino. Apparently, Aki was planning to marry Makabe in the future after graduate. On chapter 1 of the arc, Makabe and Aki are having a lunch together at the store room when suddenly Makabe ask his nickname to Aki, unfortunately Aki refuse to tell him his nickname and accidentally blurt out about her trump card to use that nickname in case she ever got a divorce.
Koiwai is Aki's maid who was always been around her since birth . She hated Masamune a lot because Aki would always talk about him. Koiwai decides to help Masamune so she can apologize for what happened 8 years ago. Another reason is that if Masamune broke Aki's heart, Aki would become more sensitive and treat her better. Later on, in the series, she just wants what's best for Aki. In the end, she helps reunite Aki and Makabe together again.
She easily accepted that Kanetsugu Gasou is the Masamune she used to love and hang out with in her childhood. Kanetsugu's target is only her family money and doesn't seem to be interested in Aki due to the fact that Kanetsugu is also a female in disguise. Kanetsugu's family used to be rich but later went poor due to some unknown reasons. She fully takes advantage of Aki's family money by giving the contract that her family should provide funds to Gasou family for being Aki's fiancee. She avoids as much as possible to talk about the past which Aki brings up only to get disappointed by Kanetsugu's answers. However, Aki is in love with Kanetsugu despite all this and she acts like a completely different person when she is with the latter.
It can easily be implied that she just didn't want to accept the fact that she is in love with Masamune. At that moment she was thinking about the childhood Masamune, Kanetsugu shows up acting like him and Aki easily accepts it. Later on, Aki asked Kanetsugu about "his" disappearance when "he" (Masamune) was young and Kanetsugu answers that it doesn't matter since it all happened a long time ago. That makes Aki's feelings towards Kanetsugu become much colder.
In the later chapter she seems to be depressed about her which Kanetsugu has not yet realized.
In Chapter 39, Kanetsugu and Aki go to an Amusement Park where they are met with Masamune who tells her about who he really is, and that Kanetsugu is an impostor. Due to Masamune punching Kanetsugu, it reveals to her and Masamune that Kanetsugu is a female in disguise.