Since today, March 3rd, is Onpu's birthday, I thought I'd take a minute to give some appreciation for this character.
When Onpu first appears she's as close to a "Dark Magical Girl" as you can get in a show this... adorable. She's selfish, using her magic for her own benefit, doesn't give a hoot about the consequences and is really blunt with no regard for anyone else's feelings. Naturally I didn't like her one bit. One episode featured a school project where each student had to do their project based on the thing they love most. She chooses herself.
She came close to making my list of least favorite characters, right along-side Oyajide. Thankfully future seasons changed that. In fact it took quite a while for her to warm up to me. The end of the first season didn't really feel like she had done anything to be redeemed. One could argue that her actions were to protect herself and merely the others by proxy. Everyone had to give up their powers to save her from the consequences of her using forbidden magic and suddenly she's one of the heroes.
So yea, it wasn't until around the second season that Onpu really started to warm up to me. We start seeing more into her as a person. Her concerns with balancing her life as an idol and her life with her friends and family, her feeling like she doesn't have as much of an impact in raising Hana-chan because she's not around as much as the others. By the end of the second season Onpu as a character has basically come full circle and when Momoko takes center stage, I began to eagerly await any focus the show gave to Onpu.
Sometimes she can be kind of over-dramatic, and even still tends to be a bit harsh and blunt, but she's blossomed quite a bit from the conceited young girl she was in the first season. A big part of her development comes from her desire to be seen as a person, rather than an object on the TV. Part of the reason she comes to befriend Doremi and the others is because they reached out to Onpu the person, not Onpu the idol/TV star.
A few of her focus episodes later on in the show deal with her having some kind of existential crisis, wondering where her career as an idol is going, if she wants to pursue acting or questioning what drove her to want to become an idol in the first place.
I feel like Oyajide's fandom-crush on Onpu is some kind of statement about Otaku in Japan.
"Onpu" is the Japanese word for Music Note. Her head is also shaped like a music note.
Without it, her bangs somehow get longer, but aside from that her hair doesn't seem all that different.
And her voice is ten levels of adorable provided by Rumi Shishido.
https://www.youtube.com/watch?v=4xG1E0kJR2s