Wing(2000)

Wing

From Sherbet Lemon:

An energetic, cheery high school girl named Garam was having a peaceful(?) school life until she runs into a cute boy name Lime. Trouble started as Lime and his best friend, Yuyi moves to her school and monster start showing up and she's the only one around who can see it. What's more, Lime happens to be a prince from another world who came to this world to find his missing bride who he doesn't even know the face of. As time passes, Lime starts to feel weird around Garam...
...And Garam feels the same except that it wasn't toward Lime but to Yuyi...

It starts as a normal school life story, but it turns to a fantasy from the late middle