When their is no official API, where did sites like maxroll.gg or lastepochtools.com get their data for characters from?
At https://www.lastepochtools.com/ladders/ you can click on character names and you can see the complete build.
At https://maxroll.gg/last-epoch/planner/ you can click "Save/Load" and then enter a account name to see all characters.
We are running a trial version of our API right now with those two sites. We will expand the functionality and access to that API as we go.