def METHOD_NAME(locale: str, case: str = "upper") -> str:
    if locale not in GRAPHEME_CHARACTER_SETS:
        raise ValueError(
            f"Grapheme character set not found for locale '{locale}'. "
            f"Supported locales {GRAPHEME_CHARACTER_SETS.keys()}"
        )
    charset_str_origin = ''.join(GRAPHEME_CHARACTER_SETS[locale])
    if case == "upper":
        # Directly call .upper() will convert 'ß' into 'SS' according to https://bugs.python.org/issue30810.
        charset_str = charset_str_origin.replace('ß', 'ẞ').upper()
    elif case == "lower":
        charset_str = charset_str_origin.lower()
    elif case == "mixed":
        charset_str = charset_str_origin.replace('ß', 'ẞ').upper() + charset_str_origin.lower()
    else:
        raise ValueError(
            f"Grapheme character case not found: '{case}'. Supported cases are {GRAPHEME_CHARACTER_CASES}"
        )
    return charset_str