A single User object with all its details, accessed via either ID, email or your custom user ID. If an email or custom ID is provided in the URL parameters, the ID field can be omitted. Note that custom data can only be reliably kept with no more than one level of nesting. If the user currently has a custom data object and one is posted, the two will be merged.
id (string, 548223f2c7036020742ff21c) ... String id of the User to retrieve.
user_id (string, 42) ... Your custom defined user id.