true character; underlying nature; inner feelings
Refers to a person's deep-seated disposition or real intentions, often hidden beneath the surface. Used in expressions like 底意地が悪い (spiteful, malicious) or 底意地を見抜く (see through someone's true nature).
Everyone already knows that he has a nasty streak.
あの人の底意地を見抜くのは難しい。
It's hard to see through that person's true character.
根性 can mean guts, willpower, or (often negative) ingrained character, but 底意地 focuses specifically on the deep-seated, sometimes malicious, inner feelings.
deeply malicious; spiteful at heart; wicked
Compound of 底 (そこ, bottom) and 意地 (いじ, disposition, willfulness). The combination suggests the 'bottom' or deepest part of one's character.