This tool inserts the Unicode combining character U+0332 (COMBINING LOW LINE) after every visible letter, drawing the underline as part of the text itself instead of relying on HTML or Markdown formatting. Since it's plain Unicode, it survives being pasted into places that strip rich formatting entirely — bios, usernames, and plain chat messages. Because each underline segment is anchored to its own character, letters with descenders (like "g" or "y") or unusual spacing in the destination font can make the line look slightly uneven or gapped between characters.
HTML underline is a style applied by a web page and disappears the moment the text is copied elsewhere; this method embeds the line directly into the Unicode characters, so it travels with the text into any plain text field.
Combining marks are positioned relative to each character by the font rendering the text, so descenders and unusual glyph widths can make the line sit slightly differently from letter to letter.
Not automatically from this tool, but you can run text through the strikethrough generator and then manually add underline marks, since both are just combining characters layered onto the same base letters.