Born to an old money family in Geosenge town, he showed strange behavior that had his parents believe he was a psychic, but later figured he was a medium of some sort, though they never got it officially tested (This Will Be Important). Yves was a troublemaker and was said to have a bit of a death wish, with how much he'd run off to dangerous situations. One morning, his parents were found dead, and he was completely missing. He remained missing for a few weeks until mysteriously showing up back in town thanks to a Trevenant.
He remained hospitalized for a long time, initially due to severe physical injuries, but also after his mental stability was shown to be very poor. He was unable to explain what had happened to him, eventually stumbling on an explanation of secret labs, aliens, and a certain famous man. His wild claims that were unable to be verified, along with his past history of "seeing things," lead him to be diagnosed with early onset schizophrenia.
He was moved into his aunt and uncle's home in Vaniville Town after being stabilized, figuring it would be good to stay with family in a quiet area. He remained a complete paranoid shut-in growing up, completely rejecting other people and being upset knowing that those around him thought he wasn't able to perceive reality.
As he neared the age of emancipation, he decided to try and act like he had been cured, as if everything he believed happened was made up in his head, in order to be able to make choices for himself in the future. He was even beginning to believe that it wasn't real. Quickly, he took on the opportunity to go on a League trip, to make his own journey for the first time.
He runs into Lysandre again nearly immediately.
Lives by the church with his family, of whom work at said church. Is a strange hermit that most people avoid.
A ghost who died awhile ago at Lysandre's hands. Was captured to be used by a doppelganger but was not considered fit for use for whatever reason, so was used for whatever other purposes Lysandre wanted (research, other things...).
He died with a severe grudge, both at what physically happened to him, and the knowledge that those at home would never know the truth of the matter. This grudge plus his strong spiritual power in life led to him becoming a powerful spirit, though as all spirits do, he eventually lost his memory as to why he's like this.
In modern times, Yveltal was able to convince Yves to work alongside him and cause trouble towards Xyla and Xerneas. Such powerful ghosts are less common in the modern world, so it's more of a struggle to deal with him. Xyla wants to help him move on, Xerneas would be happy exorcising him at this point.