Imagine the most adorable couple you can think of. Now throw a truckload of sugar on it. That's Kawaii Hito. It definitely is the perfect kind of manga if you just want your heart melted by adorableness. You have a scary looking guy, falling in love and ending up in a relationship with a cute girl. Both of them are the most pure-hearted individuals you can think of, and the manga accompanies them in their development as a couple, but all in a very light-hearted manner (no stupid over-the-top drama or love rivals, etc.). The main focus practically stays solely on their relationship, however introducing someside characters (friends, family) along the way and shedding at least some light on those too.

If you are looking for a sweet and innocent story, you'll have trouble finding something better than this, in my opinion.