Wow, Tsubasa's a really fun character. Everyone else has said it already, but she's definitely upstaging Himeno-chan when it comes to my preferences. Don't get me wrong, I still like Himeno and think she's cute, but the Tsundere "too mature for her age" girl chasing after a clueless guy trope's been done to death, even as side-characters in slice of life manga. Like in YanOta for instance, off the top of my head.
Not that I mind reading more of it, I just find Tsubasa more refreshing and interesting right now. As for those saying that the author should make her a main, I agree. She'd be great in a spinoff. Also: a lot of times creators only have rough ideas when they start a work, but when they hit their stride, they can often make better stuff (including characters and plotlines) than they had in mind at the start. So making new characters that are better and more interesting than the old ones is not uncommon.
So is revising old characters to make them better (or worse sometimes) over time, if a work goes on long enough.