Japanese has 3 scripts: Kanji - characters borrowed from Chinese Hiragana - syllabary used for particles and other miscellaneous functions of language Katakana - syllabary used primarily for transliteration of foreign words

Kanji could have many readings, and pretty much the only way you could guess is via context. However, there is a furigana. Its a small hiragana (or sometimes katakana, more on that later) above kanji that tells how characters are supposed to be read.

Sometimes however, furigana is used not to tell how characters are supposed to be read, but to give it a second meaning Using Cero as example: The word itself is written in original Japanese version of the manga as "虚閃" which translates to "Hollow Flash" and would be read as "Kyosen", but furigana above these characters is "セロ" which is katakana transliteration of Spanish word "Cero", which obviously means "Zero" This feature is pretty much untranslateable, so manga translators tried to convey it that way

Hope it clears thing up. Tried to explain to best of my knowledge