The main characters are VERY similar. They both are practically invisible to everyone with out any friends. So unseen people can't see or hear them, they feel no real presence at all. Till a day when some boys notice them. But in Koko ni its more realistic without any magical stuff, and less boys. BUT! It doesn't change that they're both similar and really good stories!
Both main characters are very similar! They try to be seen by everyone but are too shy. Their presence to so slim that even class mates or teachers oversee them.
Both protagonists are female and meet with popular boys who see them as they are.
Both are by the same author with very similar artwork. In Super Darling, the heroine meets four handsome guys who stay by her side for a purpose which is much like Barajou no Kiss! There's more fantasy/supernatural-ish stuff in Barajou no Kiss however but both are very good reads with same idea of four guys falling for one girl!