First thing that popped in my head, almost all harem anime.

I haven't seen Chaos Head yet, so these recommendations may or may not be similar to it.

Well there is Accel world, the main character was a big loser. He is fat, small, and picked on a lot. Until he met the black snow princess, the most popular girl, and learns about the accel world, where time is slowed down by an insane amount and there was a popular fighter game called Brain Burst, used to survive in the Accel World.

A good RomCom, ToraDora. The main character has been avoided in school due to his scary looking face. One day he met Taiga, a cute little girl, who is extremely violent and they found out about each other's crush for each other's best friend. There they decided to help each other and start their journey as Tiger and Dragon.

There is more than likely more that I could list off, but I chose the ones that I can think of right now that isn't a harem series.