This is the effects of being ignored, unseen, and unheard. Psychologically speaking everyone has deep needs to be seen, heard, and validated. A person stuck in your proposed state is effectively discarded from any opportunity of this. Even a momentary interaction would not meet these deep emotional needs. Children who lack positive attention at young ages are significantly more likely to show difficulties re-integrating with society. Fortunately, your character has a history of participating in the real world so it is possible for re-integration to occur, but be warned: it's a VERY long and difficult road.
In more detail, the neural pathways take about three to seven weeks to take hold, and about three to six months to "cement" or form (ie to create a habit). The converse is also true - after about six months of being ignored your brain has entered a state of being "adapted" to no longer living with human contact and attention. To learn more about how this process works I recommend reading The Mind and the Brain: Neuroplasticity and the Power of Mental Force (NB: you can safely ignore some of the grandiose claims made by the author - Dr Jeffrey Schwartz - especially about proof of "the mind"; the book does a great job explaining the science and studies behind neural pathways and changes in brain chemistry, even if you disagree with his conclusions).
As your character goes reaches into the six month without any human attention or feedback, they will begin to show signs of isolation, lack of concern for the well-being of others (no longer being shocked or disturbed by violence, for example), and a general sense of emotional "numbness" and indifference. Emotions work a bit like a pipe - it's either on or off. You can't just shut off bad emotions (nor good emotions), when emotions shut down they all shut down.
When this happens, serotonin and dopamine levels in the brain restructure to place the character into their new perceived "role" in society - either at the top or the bottom of it. They would either develop a grandiose god complex, or become deeply depressed with a continual sense of being a shit-stain on the earth.
What would follow in the months and years afterwards would move from a sense of isolation and numbness to full on detachment and dissociation. Basically, your character would "drift away" from identifying as human, and turn into "an other" - a beast of some kind, a god figure of some kind, or a deranged rambling maniac.
Studies around this are nonexistent, as there is no pool of people who become adults and then disappear entirely from society without any interaction at all, especially not for years on end. Such studies would be unethical. But in the USA, if you've been imprisoned for 20 years or more, you qualify for some types of disability benefits because the government believes you're so dissociated from society that you'll never be able to re-integrate (or that doing so is too impractically burdensome). The point being if your character stays in this place for 20 years or more, their brain is likely mush and reintegration would be difficult if not impossible.
However, coming into this ghost-like state after maturity (approx. age 25) and returning to a life-state within 20 years, reintegration is difficult but not impossible. This would be akin to someone with severe brain trauma and amnesia after an accident learning to walk, talk, read, and write again. They would be able to remember these things, but it would seem "like another life" to them. They would have to learn social interactions again from scratch. From simple introductions, to making eye contact, to smiling and showing concern for others. As a general rule, I would expect every month in the ghost-like state (after the six month) to take 2-3 months for integration. Some could do so faster, some longer, but in general if your character was stuck for a year I would anticipate 2-3 years re-integration time on average.
