U+FDEF is not a valid Unicode code point / character.
It belongs to the "Arabic Presentation Forms-A" block which goes from 0xFB50 to 0xFDFF but is not defined in the Unicode 15.0 database of code points.
This code point first appeared in version of the Unicode® Standard and belongs to the "Arabic Presentation Forms-A" block which goes from 0xFB50 to 0xFDFF.
You can safely add this character in your html code with the entity: 
You can use the u+FDEF copy pc button below.
Easy u+FDEF copy paste:
Unicode meta-data
The following table show specific meta-data that is known about this character.
| field | value |
|---|---|
| Codepoint (hex) | FDEF, uFDEF |
| Legacy name (Unicode 1.0) | - |
| Official name (Unicode 15.0) | - |
| resolved name | |
| block | Arabic Presentation Forms-A (Arabic_PF_A) |
| common typos | u+DFEF, u+EFDF |
There are alternative spelling that can be found in the wild for the unicode character FDEF like u FDEF, (u+FDEF) or u +FDEF. You can also find u-FDEF, u*FDEF, un+FDEF, uFDEF, u=FDEF or c+FDEF. You can also spell it with u FDEF unicode, u plus FDEF, uncode FDEF or unicode + FDEF.
Glyphs and symbols in your browser
The following unicode chart presents different versions of the glyph corresponding to the unicode characters u+FDEF that are available on your computer.
In order to type this character easily, you may want to download and install a unicode Arabic Presentation Forms-A keyboard.
A sample of fonts are used below to display whether the character has a glyph in this font or not.
Encodings (Unicode characters converter)
The following character table converter for +uFDEF allows you to see the value of the character in different encodings
| encoding | hexadecimal | decimal | binary |
|---|---|---|---|
| UTF-8 | EF B7 AF | 15710127 | 11101111 10110111 10101111 |
| UCS2 | EF FD | 61437 | 11101111 11111101 |
| UTF-16LE | EF FD | 61437 | 11101111 11111101 |
| UTF-16BE | FD EF | 65007 | 11111101 11101111 |
| UTF7 | 2B 2F 65 38 2D | 185478756397 | 00101011 00101111 01100101 00111000 00101101 |
| UTF7-IMAP | 26 2C 65 38 2D | 163953588269 | 00100110 00101100 01100101 00111000 00101101 |