Ai Yori Aoshi, Mikakunin de Shinkoukei Recommendations
Both are romantic anime where the main protagonist doesn't know that they has a fiance. In the anime the fiance arrives suddenly and make the life enjoyable. I recommend to watch both.
Both are about an engagement between a couple, but are supposed to keep the it a secret from others. The only difference is that Ai Yori Aoshi is a harem series, and Mikakunin is more of a romcom.
The same scenario where the mc are engage to someone in an arrange marriage, except one was more for the seinen type while the other is more for the younger ones.
Main couple that doesn't really know each other well at first. They keep their status secret from their friends. In both animes you find comedy, drama and a solid story. One is supernatural but that is not the main issue in the story. They are cute, hilarious and interesting. (Sorry for my bad English)