Precious Leaf was repeatedly sexually abused by his father around 2003, he was only 13yo back then. His mother knew because she was there when her husband did it.
He is retarded but eventually started to talk about these events and the family really didn't want their sexual abuses to be exposed, first they tried to obstain his silence buying him Pokemon games and cards but it didn't work, so they have abandoned him far enough from their house instead. This is the period when he has invented an imaginary girlfriend and as substitute for his family, she is in fact not real but he refuses to accept this truth even know that he keeps growing while his fictional girlfriend is still 11yo.
He might become a pedophile himself.
Right now Lead is nothing more than an imaginary friend or a tulpa result of his schizophrenia.
https://en.wikipedia.org/wiki/Imaginary_friend 