I had the same reaction as Fuutarou for Nino's hairstyle, but upon proper review... well, I like myself some good ponytails.
I find that Nino has been the most pushy and most disregarding/inconsiderate of Fuutarou's thoughts, be it before she fell in love and even after. From a personal standpoint, that has been a continued point of detraction. But need to give credit where due as the latest chapter did feel tempered enough, even if it feels like Nino's development hasn't paramounted on developing her as a character outside her rocky relationship to Fuutarou. Not as bad as having no development at all, but I would say I am tired of people calling any change in personality or story direction to be "character development," but I may just be being over-critical when we all sometimes pick at small details to full our passion for a favourite character. I still stand by the thought that among the five, I'd find Nino the most unsettling to win the race.
One thing to point out, I find the anime better introduced Nino than the manga. This may be due to the author not having a clear goal of how to take her, or didn't have one during the pilot/first chapter, but in the anime, a few nuances and interactions were added to support her identity as the more... "volatile" sister of the five; you get to know what's coming. That didn't feel much present in the earlier chapters of the manga, so her acts of getting rid of Fuutarou from being a tutor felt dissonant to the other times she cooperated to study. Her latter mention of her goal being to keep the family together had been just weak material in propelling her character.
@Arcversion I certainly like your point about Ichika, as she has been the one character who ping-pongs between identities as a character. Is she supposed to be a strong and flirty but in some odd way responsible onee-san? Or is she meant to be an overwhelmed, trying too hard actress sister? OR is she pushing much more and is a romantic mess? Ichika has been a bit of a blank slate in what her actual personality is, but she also wears "masks." Some are convinced these "masks" are just part of her being an actress, but it's a false herring or simply shallow to think the job or their talent makes the character.