Brothers

Once upon a time there was an invisible man who was always forgotten and unseen and a woman who could see everything about a person emotions, thoughts, and most importantly the person themselves. They fell in love and got married and had two children-twins but the mother died bringing them into the world but not before naming them the older one Nagisa, and the younger one Tetsuya they grew up together at an orphanage but Nagisa got adopted and Tetsuya got adopted a few days later

Or~ a crossover which Nagisa and Tetsuya are brothers