pronoun
Poetic or literary pronoun for 'self'; used in elevated or reflective contexts, not in everyday conversation.
己を知ることが大切だ。
It is important to know oneself.
Humble first-person pronoun used mainly by men in formal or self-deprecating contexts; can sound old-fashioned or stiff.
己が未熟なばかりに、ご迷惑をおかけしました。
It was due to my own inexperience that I caused you trouble.
pronoun
Derogatory second-person pronoun used to address someone with contempt or anger; often heard in confrontational or rough speech.
己、よくも騙したな！
You bastard, how dare you deceive me!
adverb
Adverbial use meaning 'by oneself' or 'of its own accord'; often appears in set phrases like 己から (of one's own will).
彼は己から進んでその仕事を引き受けた。
He took on that task of his own accord.
Standard reflexive pronoun for 'self'; neutral and widely used, unlike the poetic/literary 己.
Casual first-person pronoun for men; 己 as 'I' is humble and formal, while 俺 is informal and assertive.
Common second-person pronoun that can be rude; 己 as 'you' is more explicitly derogatory and confrontational.
Derived from Old Japanese おの (self) + れ (nominalizer/suffix). The exact historical development is uncertain, but it has been used as a reflexive and personal pronoun since ancient times.