As character names:

U+6FFFF <not a character> character (&#x6FFFF;)

As raw characters:

񯿿

As a string of HTML entities:

&#x6ffff;

Decoder output:

Byte number 1 is decimal 241, hex 0xF1, octal \361, binary 11110001
This is the first byte of a 4 byte sequence.

Byte number 2 is decimal 175, hex 0xAF, octal \257, binary 10101111
This is continuation byte 1, expecting 2 more.

Byte number 3 is decimal 191, hex 0xBF, octal \277, binary 10111111
This is continuation byte 2, expecting 1 more.

Byte number 4 is decimal 191, hex 0xBF, octal \277, binary 10111111
This is continuation byte 3, expecting 0 more.

U+6FFFF	<not a character>