After seeing what other people have to say about Yuuki, and as someone who just caught up on this manga after reading it for the past week or so, Yuuki getting introduced actually felt pretty natural to me. The story was starting to get stale, and there weren't many places for it to go if the status quo was upheld. (There's only so many times you can have chapters dedicated to just Manabu getting flustered over Amano showing him her panties by accident in a row).
Yuuki being introduced is a consequence of Manabu being more honest with himself, and becoming more outgoing as an individual. He's not just some shut-in that tries to keep to himself anymore; he's actually going out there and expressing himself to more people than just Amano now. So it stands to reason that someone besides Amano can take an interest in him. Kindness is attractive (unlike simping, looking at you, Ryou the yandere)
