Problems related to modeling (patterns).
This needs to be a pattern, with the correct operations defined.
Say that a city has a population of 10,000 people. The main character has interacted with some of them, and they then have a distinct personality. They are still a member of the city, but are a distinct individual. There is no need to directly model each individual in the city, but instead model them as a group.
If this pattern is to be used, rather than actually modeling the whole city population, then there need to be some rules that are followed with this. For instance, there cannot be more than the population size number of individuals. As each individual is branched off, it needs to be correctly integrated with the rest of the population for correct behavior.