How to make a custom character on a TFT LCD display? (128x128)
Simple: you don't.

You only need to create a "custom character" on character-only displays (where you are actively programming a new character into the display's internal character set).

With a graphical display there is no "character" - only graphics.  The graphics library has fonts, which it then uses to convert text into graphics.

To display something that isn't in your currently selected font, just draw it. That could be from a bitmap graphic (there will be examples in your chosen graphics library) or using primitives, like circles and lines.