The first character in the Unicode Private Use area, or "U+E000". It is often used to display specific non-cross-platform characters in specific
operating systems or in specific applications.
Sometimes it is used to display the Tux logo in Linux (
Libertine Font). On twitter, it is used for the left down arrow. It is not a null character ( ) and it is not an Emoji that isn't being displayed correctly (). The box with some letters and numbers in it (hexadecimal code point) is sometimes referred to as "tofu".