For a character to be built up, there is nothing like an environment for them to develop in.
Whether it’s a city, a countryside, a prison, or somewhere desolate, environments are always crucial in making a character. It also helps to have other characters in these environments. While they are not always needed, they do help. Environments and settings are what give readers a bigger interest, and therefore more likely to want to see the outcome of characters so long as they remain in these environments.