The Umbra Viventem — a series by Everflowing Fountain Pen
Part of the The Otherworld universe

One day Roy, a young boy, finds a strange creature abandoned in a box. Turns out the creature is Mitsuki, a cute girl with powers that defies all human understanding. Roy decides to take Mitsuki home and let her live with him. After years of living together he starts to realize he is being given incredible powers. More strength then an Olympic athlete, a mind greater than any genius, he becomes better and better with each day. But the most serious thing he gets is a hardon every time he sees Mitsuki's sweet smiling face. A story of Love and Lust as two lovers from different worlds try to understand each other.