Unicode has the concepts of code unit (1 byte in UTF-8) and code point (1--4 bytes in UTF-8). It does not clearly define "character"; a glyph can consist of several code points. UTF-8 works great with Forth-94: Use strings to represent glyphs (or sequences of glyphs). Therefore the xchar words see little use, but they are there if you really want to deal with code points.
M. Anton Ertl (2025) studied this question.
Synapse has enriched 5 closely related papers on similar clinical questions. Consider them for comparative context: