Can a person be 'themselves' in total isolation? Not interacting with anyone and doing nothing? I'd say- not so much. We get an idea of who someone 'is' by how they react to certain things. Can we predict how we would react to a situation that's going to happen today? Maybe- to some extent. Do we know how we'll react to the same situation in ten years time? Maybe not as reliably. Stuff could have happened in our lives that changes how we react to things at that point.
As simple as- a song we might like now, we may hate in ten years time because by that point, we have negative associations with it. Say- it was playing while we heard some very bad news. It may be so bad that we burst into tears yet, at the moment- we're not outwardly emotional people.
I'm not sure we can even predict our own behaviour in the immediate future all that accurately. People who would like to think they are brave and good may end up bottling it if something horrific happened or, vice versa. Some things, we simply haven't been tested on. Maybe, we'll never be tested on them.
I've always wondered that with something like religion. Who's the 'stronger' or, more righteous person? One who is massively tempted to sin. Who maybe has sinned in the past but then, uses all their fortitude to resist or- someone who has very little desire to sin in the first place? Surely, they're setting out with an unfair advantage. It's hardly an achievement to resist something you didn't even want to begin with! I've always wondered- if there is a God- how do they work out the scoring! Bearing in mind also- it's God's plan that we desire so much to begin with. (I know some will blame it on an apple but- God created the apple and the massive temptation to eat it...) If there is a God anyway.
I think it's more that we have a hope or fear of who we really are and we constantly do stuff to either prove or disprove it. I've done things or rather- not done things in the past that I don't think fit with who I hope I am. Maybe that has made me reassess who I am to some extent. They've been unpleasant things. I haven't been there for people when they were there for me. Even worse though, I've made excuses for that to myself rather than admit that maybe, I'm not as kind as I hoped I was. I even decided that- yes, maybe I'm not that altruistic person but, it's not my fault! Which is worst of all really.
But no, I find it unlikely that many people know themselves entirely- unless they are unwaveringly good or, consistently bad!
Again though- is someone who is 'good' necessarily 'good' deep down? What if you do kind things out of politeness and then, slightly resent them afterwards? Maybe there's good reason to. Maybe someone helps someone else with a project and then, they utilise all their time and effort without giving them any thanks or credit. When they ask for help again- would that person still be 'good' if they made an excuse that they were busy? (When they weren't.)
Everything that happens in our lives changes us. Every interaction we have with a person either cements our idea of that person and our relationship with them or, calls it into question. Someone who behaves dishonestly probably doesn't deserve our complete trust in the future. Does that make us a bad person? Not necessarily, but it can make us overly cynical/ pessimistic.
As such, I think it's more that we start out with a baseline character- from our genetics and upbringing and events thus far. But then, various events in life can change that- almost entirely sometimes. I've known of the biggest bullies in school become therapists!