Mahiro grew up with an inferiority complex. People always chose her twin sister over her. Aoi grew up with 3 younger siblings to care for after his mother passed away. He always blamed himself a little for her death. Both Mahiro and Aoi save each other from these things that are eating away at them. Their love is super cute. The love square is a bit annoying, but thankfully, helps with character development.
Chikyuu no Owari wa Koi no Hajimari