The character █ (Full Block) is represented by the Unicode codepoint U+2588. It is encoded in the Block Elements block, which belongs to the Basic Multilingual Plane. It was added to Unicode in version 1.1 (June, 1993). It is HTML encoded as █.
There is a separate block character, usually a perfectly square one, that is shown when your system cannot display a unicode character, however this block character is rectangular.
That persons username is MaxVoltage. They probably set their display name to full blocks. Some apps display the display name and others display the username. That’s why apps show one thing and others show a different thing.