Having one character can be difficult because it makes the entire story fall on the shoulders of one person. Also, if there is only one person, they have no one to interact with. That's another disadvantage for you. You can't fill the plot with witty banter or funny conversations because there's no one to talk to.
An easy way around the "having no one to talk to" issue is by having the character talk to themselves in an attempt to keep sane. Or the stress starts driving them into insanity and they start hearing things.
Maybe he scrounges up a bunch of rations and starts to hear his wife's voice, laughing at them for having food on their face. Maybe he hears the voice of his disapproving father scolding them for not being able to get out. Occasional flashbacks to a time before the MC got trapped here would help flesh out his character and make him more sympathetic.
The notes from the previous denizens provide interesting clues as well. It's a mystery for the MC to solve bit by bit, and each new note or clue tells them more about the people who used to be here, characterizing them further. The MC's reactions then characterize him further.
For example, let's say one of the denizens was named Clara. She has perfect handwriting that loops and curls and she writes tons of hearts on her notes, but she also has an oddly dark, almost mocking sense of humor. She's a trickster.
Example 1: "Make sure to feed Bob before you go out. Wouldn't want him to eat another friend, now would we?" the first note says.
Example 2: At the bottom of the stairs you see a note that says. "Watch your step. Remember, every even-numbered step is safe." After tripping over five trapped stairs and eventually making it to the top you see a note that says. "Whoops, did I say even numbered stairs? I meant every third step. My bad! I hope you enjoyed your trip!"
Then the MC's response could be. "I'm glad I never met her in real life. She'd probably stab me in the back with a smile on her face."
Lastly, you can characterize the monsters as well. I see no reason why monsters cannot talk. Perhaps they taunt the MC at every turn. It could be simple stuff like "We want to eat your flesh." Or something a little more complicated. "Come out, come out, wherever you are! We know a thousand ways I could tear you limb from limb, and we want to try them all!"
Or you could do both and have the twist be that the old inhabitants are the monsters and show their slow decline until they eventually got like that, hinting the MC could also become one of them. Or perhaps is even already a monster himself and does not realize it yet. He might have even been the one who started the mess in the first place.