ordinary person; average person; nobody special
Almost always used in negative expressions like 只者ではない or 只者じゃない to mean 'not an ordinary person', implying someone is remarkable, extraordinary, or suspicious.
彼の腕前は只者ではない。
His skill is not that of an ordinary person.
あの男、只者じゃないと思った。
I thought that man was no ordinary person.
普通の人 is a straightforward, everyday phrase for 'ordinary person', whereas 只者 carries a stronger nuance of being remarkable when negated.
Standard kanji spelling.
Mixed kana-kanji spelling; common in informal writing.
Rarely used kanji form.
Compound of 只 (tada, 'ordinary, mere') and 者 (mono, 'person'). The reading ただもの is a straightforward combination of the two elements.