Runako, sensei and Oriana don't have a chance unless there's a shitty harem ending.
Runako is 'perfect' and is pretty bland. She wants to be an adult before her time, and aside from career aspirations, that's an obviously childish character trait. Shio has no push or pull, yamato nadeshiko ass so she's pretty boring, despite her "main heroine" energy.
Therefore, for me, there's only Akari (who somehow seems to have the grandest chapters, probably because her personality enables more fun stories) and Haru, who, despite everything, is certainly not boring.