Why does 定命 have the furigana さだめ in the Oreimo: Kuroneko manga?
As described in this answer on the Japanese Language SE, there is a phenomenon known as &quot;ateji&quot; where the kanji and associated ruby* are not 100% aligned, either in reading or in meaning. This particular style is especially used in light novels and manga as a way to provide a double message - the ruby is the actual word spoken, but the kanji gives a clearer understanding of the desired meaning.
*A Ruby character is any smaller writing placed above the main writing, technically it's only furigana if it actually provides a guide to the reading of the kanji but that's a distinction that is frequently ignored.