1.
willpower; guts; determination
Strong mental fortitude and perseverance. The ability to push through difficulties without giving up.
根性のある人だ。
That person has guts.
根性で最後まで走りきった。
I ran all the way to the end through sheer willpower.
あの選手は根性があるから、簡単にはあきらめない。
That athlete has determination, so they won't give up easily.
2.
temperament; nature; disposition
A person's fundamental character or nature, often used in a negative sense to describe an unpleasant disposition.
根性が悪い。
To have a bad nature.
あいつは根性が曲がっている。
That guy has a twisted personality.
根性の腐った人間とは付き合いたくない。
I don't want to associate with people who have a rotten character.
Sense 1 is positive, praising perseverance and fighting spirit. Common collocations: 根性がある (to have guts), 根性で頑張る (to persevere through willpower). Sense 2 is negative, describing someone's unpleasant character. Common collocations: 根性が悪い (bad-natured), 根性が曲がっている (twisted nature), 根性が腐っている (rotten character). The compound 根性論 refers to the belief that willpower alone can overcome any challenge.