Ai Sunakawa (砂川 愛, Sunakawa Ai) is Makoto's older sister and a college student. Takeo mentions that she, like her brother, is a great and kindhearted person.
Ai is considered a very beautiful woman. She looks a lot like a female version of Makoto. She has long wavy red hair, brown eyes and wears very stylish clothing that compliments her figure.
Like Suna, she is a kind person. While a calm person, she is more in tune with her emotions, becoming emotional fast and being a very expressive person in general. She is very intelligent and observant, like Suna. She has had feelings for Takeo since they were in elementary school, and still shows some distinct attraction for him, as exhibited when talking about him with Yamato. While somewhat confrontational and harsh, she is actually a very sweet person who looks out for others, especially those she cares for, and is extremely helpful and selfless. She is also very responsible, not liking it when others slack off and scolding those who she feels need to get their act together.
She is Suna's older sister by three years. Her father was worried when she was born because he had a weak heart and hoped she would be okay. When he learned she was born healthy, he was ecstatic. She grew up in a loving home. When Takeo was younger, Suna's sister would treat him like family and watch out for him, telling him his clothes were backward and wiping his face.
When she was younger, she was tall and thin and compared to an ugly statue called "the Image of the Growing Child" which was at the courtyard of her elementary school and the boys bullied her, nicknaming her "The Growing Statue Girl". However, Takeo didn't think she looked like that but instead believed she looked like a white lily (he didn't know the name of the flower but pointed to it) and that she was beautiful. At that moment, she developed feelings for Takeo. She decided to wait until he became a university student to confess her feelings due to their age gap, but before that time could arrive, Takeo had already begun dating Yamato.
Although they are siblings and share some physical characteristics, Makoto is almost the polar opposite of Ai. The two express their care for others differently, as Makoto shows his concern subtly while Ai expresses herself boldly. Nonetheless, the two are close, which can be seen whenever Ai says "welcome home" or "I'm back" to Makoto whenever he comes home, and he would be happy to see her.
Makoto cares for Ai, which was specifically shown when he acted as her confidante regarding her feelings for Takeo. Even though Makoto is Takeo's best friend, Ai trusts that her brother would never tell their neighbour about her feelings. Learning of Ai's feelings for Takeo demonstrated the siblings' honesty with each other, and their overall closeness.
Considering how observant Makoto is, he surprisingly did not know that Ai had a crush on Takeo. Although it could be that Makoto did not expect Ai to develop romantic feelings for their neighbour, and believed her actions toward him were like an older sister’s. Thus, despite the honesty the siblings established, they can keep secrets from each other. (For instance, Makoto never told Ai that he and Takeo kissed.)
Makoto did find Ai annoying when she cried and shook him. Ai is the only person throughout the series that Makoto thought of as annoying, which also emphasizes how close they are. Seeing how expressive Ai is may have influenced him to avoid behaving like her.
Takeo - Ai has always been a big sister figure for Takeo, ensuring he's dressed properly in public and not forgetting anything important. It's revealed that she has been in love with him for a long time, ever since Takeo was in third grade. When she discovered that Takeo actually has a girlfriend, she was in shock from a combination of disbelief and jealousy.
Yamato - Ai is initially suspicious of her, thinking that no normal, pretty girl would be interested in Takeo without underlying reasons. After some misunderstandings were cleared, she understands that Yamato is just as naive and caring as Takeo and approves of their relationship, in spite of her own broken heart.
Hayato Oda - Ai and Oda are both freshman at the same university. They first met when Oda misplaced important documents for a seminar they were both attending. While other girls were sympathetic towards Oda, Ai yelled at him and told him to apologize to their teacher. She accompanied him and later found the documents that he had misplaced, resulting in Oda falling in love with her. When under hypnosis, Oda witnessed Ai confessing her feelings for Takeo in front of a small crowd of university students. He pushed everyone aside, telling them that it was enough and brought her out of her hypnosis. Following this, he followed her from Nagoya to meet Takeo since he wanted to know what kind of man Ai had fallen in love with. He later attempted to get Ai and Takeo alone together during their outing at MM land so Ai could confess to Takeo. In the end, she refused and told him she'd never confess. Ai doesn't particularly like people who are pushy, make excuses or are slack, so Oda tends to get on her nerves.However, she doesn't seem to completely hate him and in a way almost takes care of him. By volume 8, it is shown that she is beginning to get over her feelings for Takeo and is starting to fall in love with him.
- She states she is not a big fan of sweets in episode 23.