Why/how did the MC get isolated from society?
Did they turn their back on society, or did society turn its back on them? Did they do something unforgivable and flee? Or were they unfairly persecuted and grew to hate society in return. Or was there some accident (e.g. a shipwreck) that caused the isolation? The MC's motivations will be colored by the past that led up to this.
What does the MC want, and what do they need?
Do they just want to be left alone? But maybe they need the warmth of friends and family that society can offer. Maybe they want to escape their destiny, but need to go save the world. Maybe they want to hide from the shame of what they did, but need to redeem themselves. Or maybe, to be contrary, they want to be part of society (but are afraid to) and need to learn that it cannot offer what they thought.
What is the inciting incident that sets the story in motion?
Is society encroaching on the hermit's sanctuary, forcing them into action? Is the past catching up to them? Is there an accident that requires them to get help or supplies and thus leave their isolation? Is there someone they need to save (either due to duties from the past, or because the victims are right here in front of them)?
How should the story end?
Does the hermit reintegrate with society? Or start a new society (or family). Or do they fix some problem just to disappear again? Or do they die redeeming themselves for some past crime that they hid from outside society. What's appropriate here depends on the hermit's past.
What is the setting?
What naturally comes to mind is the picture of a hermit living out in the wilds far away from humanity. But it's also quite easy to be isolated right in the middle of a populated city - you can have everything delivered and barely ever interact with anyone. The setting might also be post-apocalyptic, with abandoned cities where one can live without being bothered by anyone. Or science-fiction, with whole planets to be alone on. Or perhaps the hermit is a wanderer, rather than stuck in one place.