noun
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).
彼の底意地が悪いのは、もうみんな知っている。
Everyone already knows that he has a nasty streak.
あの人の底意地を見抜くのは難
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.
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.