콘솔 확인

selectCharacterElm.addEventListener('click', function (e) {
      console.log(e.target.getAttribute('data-char'));

});

주요 CSS

body[data-char='ragirl'] .select-character-btn-ilbuni {
    opacity: 0.5;
}
body[data-char='ilbuni'] .select-character-btn-ilbuni,
body[data-char='ragirl'] .select-character-btn-ragirl {
    opacity: 1;
}

JS 코드

selectCharacterElm.addEventListener('click', function (e) {
        const value = e.target.getAttribute('data-char');
        document.body.setAttribute('data-char', value);
});

끝!!!!!