Like the a shark just under the water's surface, the yuri is there... lurking. :3
Does this mean you're more likely to die from yuri than getting struck by lightning?
It should at least have a Shoujo Ai tag. It is very clear that Ran leans that way and the Valentine's Day chapter certainly indicated that they share mutual feelings that go beyond friendship. I do wonder at the fact that they had that exchange and just... let it go...
I think it's partially the format, but it's also the way the characters are written. I hope I can remember things clearly, I don't feel like re-reading every bit for an impromptu analysis of the story:
Through they express it in different ways, neither of the main characters are the best with romantic relationships, or properly expressing their feelings.
Summaries: Keiko n' Ran fear their past experiences, and losing their present comfort-zone.
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Keiko wades through unhappy bonds (family, exes), bottles up her emotions, and sorta puts off the idea of serious attachments for the future. While she has engaged in horseplay and sees Ran as attractive, she doesn't seem used to the idea of Ran as a possible lover. She has grown to known her as an eccentric and very close friend, not as someone she wanted to ask out on dates.
It should also be considered that she has only had heterosexual relationships before, and she seems unhappy with the intense reliance and sharp break-ups she had with her previous lovers (including the tragic one with her finance.)
Seeing Ran romantically, rather than in a familial way, could seem challenging to her; as something that could upset this relatively stable, happy, predictable period of her life. She could be afraid that Ran would become like her boyfriends, something that's lost in the past.
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While Ran is more straightforward in her goals, she's also a bit messed up. While she has a long-term attraction to Keiko (it's interesting to see her previous antics as less "joking" and more "testing the waters"), it was difficult for her to work her way up to a very sanitized, very high-schoolerish way of confessing her feelings. Which might be noted as a call-back, as she sorta fell back into Keiko's life after they said goodbyes in high school, and she at least admired her in a platonic manner then.
Even after the confession, she and Keiko are in this strange border state, where they are still friends and roommates that gently test boundaries, doing small things like holding hands.
And there's other elements, like her alcoholic tendencies. But really, the main thing is that she's gregarious while being a bit hesitant and closed-off at heart. Which, I guess might be partially due to both the dead dad (people can just die, no matter how much you love them) and partially the general adult life thing (sometimes, you can just fall out of someone's orbit and lose contact.) She could be afraid of working for a lost cause.
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I apologize for the wall of text and over-analysis of characters I'm a fan of. Is this the feeling some nerds describe as "yuri hell"?