Blank name

Copy the blanks and paste them in the nick. See which one actually disappears in the game.

ㅤ

The blank sits in the field. Select it and copy if the button fails.

Hangul Filler

Code point: U+3164
HTML: ㅤ
Unicode class: Lo

Other blanks

⠀ U+2800 Braille Pattern Blank
U+200B Zero Width Space
U+00A0 No-Break Space
U+2003 Em Space
　 U+3000 Ideographic Space

Test the blank in the nick field before you pay

Read the counter on the rename screen before you confirm.

Copy the first blank from the list above. Not the spacebar.
Open the nick field and paste into the nick or display-name field — not the account id.
Paste once. Do not confirm yet. Watch the field for two seconds.
Letters vanish, the field shakes, or Confirm stays off: that client refused the script. Do not pay. Walk the fallback list below.
If the letters stay, read the counter, then confirm once. Check the the nick field lobby list and a friend’s phone before another rename.
A box on someone else’s screen is their font. An empty slot after save means the filter stripped the letters after Confirm.
If you see a square, copy the next blank and paste again before you confirm.

Questions

Japanese nick instead?

Use the Japanese the nick field path. Generate kana, then test in the same rename field. If kana will not save, the fallback there is latin plus ツ — not more blank.