They arent too similar but I found the cast of Nagi no Asukara and Shinsekai Yori to be similar in appearance and personality - more so the guys then the girls.
Satoru + Hikari - the main male lead. A little hotheaded and rash.
Tsumugu + Shun - Second male lead. More dependable, reserved, and good at everything.
Manaka Mukaido + Maria Akizuki - same VA, goofiest and most carefree.
Chisaki + Saki - first similar names, multiple hair styles - both being the second darkest haired in the group, the more serious and reserved girl.
