To anyone looking for something similar to Mysterious Girlfriend X, Sundome is like that. Except, it's more hardcore, ecchi (in a cringy way and it's almost borderline Hent*i), and darker. if you passed those cringy stuff, You'dd see the bond and unusual relationship growing between the 2 main characters of that series ( ;w;
I don't really recommend reading it tho. I just found the depth of their bod/relationship quite similar in MGX ( ;w;