After rereading the entire series, I realized something: the way to tell which girl Rintaro likes is to find the one he tries to avoid the most. The girl that stirs the most feelings within him will be the one that makes him the most uncomfortable, whereas a girl he sees as just a friend he won't have any problem with. E.g. it was easy for him to spend time with Meru because he saw her as a child. Rintaro has always avoided girls, so he wouldn't know how to recognize his own feelings toward them. He could feel a pain in his heart and think it was his heart condition when it actually was love.
Hands down, the girl that Rintaro avoids most--and literally has to bolt from even when she isn't trying to do anything--is Ayane.
Reasons why Ayane will likely be his partner in the future:
- She's afraid of his 'eel', making her the only girl with a sex aversion. They're both scared of each others' bodies, meaning they'd both want a platonic relationship; they're a perfect match. (Perhaps she too has a medical condition?)
- She's the most likely heroine of this series. It's clear if you reread ch. 2:
Spoiler - She starts off the weakest dragon and a little needy, but she's diligently training throughout the series and learning Rintaro's techniques. All while Rintaro is learning about and getting used to girls. The two of them are growing together.
- She's the most righteous and kindest girl. She loves him purely and is the most concerned when he gets hurt. Plus they're in the same class year and he's spent the most time with her.
Spoiler - The way the author drops hints that Ayane loves Rintaro seems to foreshadow a big confession or something later:
Spoiler - Ayane's constant chasing after Rintaro may be annoying, but it's the key to forging a bond with him. Despite all his efforts to avoid girls, she's gradually getting closer to him. Kyouka had a moment with Rintaro, but nothing has come of it because he hasn't even met her again.
Ayane was the first girl to reach out to Rintaro. It's possible that if she hadn't, he would have transferred schools at the start--and then had to live the rest of his life as a hermit in fear of women. - Ayane dresses modestly, isn't sexual, and has never tried to forcibly touch Rintaro. If she knew about his condition, I could see her going as far as dressing as a boy to still be with him.
- What about Kyouka? She is kind of a bad girl, is too sexual for Rintaro, and will graduate this year. Plus she's more interested in having a battle partner, and since she knows his secret she knows relationships are off-limits with him.
Spoiler
Ayane also hinted at what may be the key to solving Rintaro's problem. What can kill Rintaro isn't girls per se, it's sexual excitement. When Ayane had to deal with nudity (ch. 2, in the bath), she referred to it simply as the human body. If Rintaro can view girls' bodies not as something sexual but just as human anatomy, then he can be near them without any harm. Granted that's not easy for an adolescent boy, but he's trained in calming his mind and has always avoided sexual desires. He was able to touch Kyouka because he was focused on performing together with her as a person.
Interestingly, Ayane's upbringing was the reverse of Rintaro's; he was isolated from the opposite sex while she was surrounded by them (her family is full of men). So she is accustomed to seeing male bodies (well, other than his!).