Follow Your Passion: A Seamless Tumblr Journey
Headcanon that while Nico can dissapear in the shadows if he abuse so much his powers,Will can disappear in the light once he used too much his powers.
Image a Will,that worked all day in the infirmary and abused his healing abilities to the point his hands start to glow,and he start little by little to become transparent untill he is literally incorporeal. And the only way to stop this process is having enough darkness to make sure that Will can find a support to hang on and not become light,since the dark stop the process. So Nico use the shadows to wrap them around him (and control them so they don't try to eat Will like they do to him) and make sure that,little by little,he come back to being corporal again.
Viceversa,if Nico is slowly dissapearing in the shadows,Will can just keep them away from him with his presence (son of the sun god after all),and make sure Nico is safe from them.
They are soulmate your honor.