In verse 56 of chapter 27, we understand clearly enough that there's somebody unmasking Peter as having been one of those "with" Jesus.
She's not a credible witness, exactly, since she's a female. She's surely young. There may be other problems with her trustworthiness. And so we read the narrative as if she's more or less another prop, a nameless one, there to irritate Peter, to provoke one of his three denials of Jesus before the cock crows.
Luke uses the Greek word παιδίσκη.
From Choeroboscus we get explicitly that the Greek phrase is the feminine counterpart for little boy and the the implication that it is indeed a diminutive.
From the old old Epidemics 2, 4-7 of Hippocrates we're able to read a reference to the twelve years of age of such an unnamed young girl, παιδίσκη ... δωδεκέτις.
From Menander's old Heros (The Guardian Spirit) we're able to see how sometimes such young girls, unnamed, were sex slaves of older men.
From Philo we get the contrast of phrases for females in this short set of sentences:
Σάρα δὲ ἡ γυνὴ Ἀβραὰμ οὐκ ἔτικτεν αὐτῷ. ἦν δὲ αὐτῇ παιδίσκη Αἰγυπτία, ᾗ ὄνομα Ἄγαρ. εἶπε δὲ Σάρα πρὸς Ἀβραάμ·
ἰδού, συνέκλεισέ με κύριος τοῦ μὴ τίκτειν, εἴσελθε πρὸς τὴν παιδίσκην μου, ἵνα τεκνοποιήσῃς ἐξ αὐτῆς.
Here the παιδίσκη is named (Hagar). She has a race or nationality (Egyptian, not Jewish), and she is the possession of another named female (Sarah), who is the wedded woman, or the wife, of a named male (Abraham), Σάρα ... ἡ γυνὴ Ἀβραὰμ.
For the story Philo is relating, the contrast between παιδίσκη and γυνὴ is both appropriate and expected in the narrative.
For the story Luke is relating, the contrast is highly unexpected and perhaps inappropriate.
In verse 57 of chapter 27, we hear Peter calling this παιδίσκη a Γύναι. Why?
Just for context we already know how odd this is in the whole of the New Testament. Elsewhere I've observed this:
Then comes the New Testament in Greek and its few odd uses of Γύναι /Gynai/ for direct speech to or at a woman: the first Pauline epistle to the Korinthian readers has it once; Mark’s gospel does not have it; Matthew’s gospel puts it in the mouth of Jesus once; Luke’s gospel has it once in the mouth of Jesus and once in the mouth of Peter and no more; and, except for the odd gospel of John (which uses Γύναι [Gynai] six times), this odd Greek does not appear anywhere else in the post-LXX Christian scriptures.
And so what are we English translators to make of this?
Is Peter calling this unnamed little girl, "Little Lady"?
Is he saying at her, "Woesome Woman"?
Is pointing out that she's some man's "Wiley Wife"?