夜の下で待ち合わせ [Yoru no Shita de Machiawase] Series by Maki Miyoshi

Our heroine, whose name no one in school can remember, can’t decide what’s weirder—that the most popular guy in school, Koga-kun, wants to be her friend…or that he insists he’s an alien. But one thing’s for sure—he knows her name, and doesn’t seem ready to let her go through high school anonymous! A new title from the author of My Boy in Blue!

After running away from home, Koga-kun and Miyako finally reveal their true feelings, and become an actual couple... or so they thought. Along comes another alien, here to take Chiro-chan home. Which actually causes Koga-kun to lose his memories! That went south real fast! Miyako's resolved to fight tooth and nail to ensure their happiness, but what will become of them?!