What if Nino already knows Fuutarou is Kintarou? If I'm not mistaken, Nino first saw Kintarou from Fuutarou's picture with childhood girl and since he just told Nino about 'Rena' it wouldn't be too weird if she connected the dots.
I guess the main issue here is we don't know how much Fuutarou...
I agree, I really like the artstyle but the manga is a bit annoying...
I find it interesting enough to keep reading, but nothing so far has really surprised me in terms of story. Everything is pretty cliché so far.