Using exclusively unicode characters like inverted pentagram or facsimile symbol in their names rulebreakers are able to hide their username everywhere except for in the output logs. Attempting to report anyone with a seemingly blank name like this in-game results in an error.
This makes it difficult to report offenders using this technique as you have to resort to a manual report. From what I understand the game cannot render exclusively unicode names with exotic characters. We require a character whitelist that allows common letters and symbols including non-latin scripts like kanji and cyrillic to prevent this misuse of usernames.