Gitea UI uses username to reference users mostly but at some places it uses the full name instead of the username of the user. Like for example, the full names are used in the commit history, reaction references (tooltip), etc. Whereas the usernames are used in issue discussions, pull requests, etc.
I personaly find it very confusing, and quite a few times made the mistake of thinking a user is different (from another user) only to notice later that I was actually looking at the full name rather than the username and both the users are same.