Will is a ghost. Lets get that out of the way.
Nico doesn't have a soulmate. He's sure of that. While everyone he knew had a clock on their wrist, counting down to the day their soulmates died, Nico's was always frozen at zero.
He was teased, and he didn't mind. It was kind of funny anyways, in a dark way. It was like he was a broken clock. He was truly broken in every sense of the word.
Nico has always been able to see ghosts. Or sense them, he is a more accurate description. They never appear to him in a humanlike visage.
When Nico moves into a new house with his mother and his half-sister, it gets a little weird when Nico meets a cheerful ghost boy who just happens to be his soulmate.