noun
true nature; real character
Used almost exclusively in the set phrase 得体が知れない (something of unknown or suspicious nature). Rarely appears outside this expression.
あの男はどうも得体が知れない。
That man is somehow suspicious; I can't figure him out.
この生き物は得体が知れなくて気味が悪い。
This creature is creepy because its true nature is unknown.
正体 also means 'true identity' or 'true form', but it is used more broadly and can be neutral or positive, while 得体 is almost always negative or suspicious and appears mainly in 得体が知れない.
The kanji 得 (acquire) and 体 (body/form) combine to suggest 'the form one gets', but the exact derivation is uncertain. The word is now fossilized in the negative expression 得体が知れない.