How formal is 唯一?


If I wanted to say "She is his only friend" I could say 「彼女だけ(が)彼の友達だ」or
「彼は彼女しか友達がいない」right? What about 「彼女が彼の唯一の友達だ」? How common would that be?




唯一 is relatively rarer and more difficult than しか～ない or ～だけ, and it's not a
word kindergartners are likely to use. But once you've become a teenager, it
can be safely used both in formal and casual settings. Even when used in
casual conversations, no one would think it's particularly stiff, archaic,
poetic, etc.

