her
I showed her her message.
The sentence describes giving a particular woman the message that belongs to her.

Meanings (pronounced /hɜːr/)
the object form of 'she' — used as the recipient of a verb's action or after a preposition
- I gave her the book.
- We saw her at the party.
indicating something belongs to a female person
- Her car was parked outside.
- She forgot her keys.
Word origin
From Old English hire, the dative and possessive form of hēo ('she'), from Proto-Germanic *hīz. English collapsed multiple Old English forms (dative 'her', accusative 'hīe', genitive 'hire') into a single modern 'her', making it serve as both indirect-object pronoun and possessive determiner.
Fun fact
English collapsed multiple grammatical cases (dative, accusative, genitive) of the Old English pronoun 'hēo' into a single modern 'her.' Old English speakers would have used three different forms; modern speakers use one. Most pronouns underwent similar simplification — 'him', 'them', and 'whom' are the only modern English pronouns that distinguish object form from subject form clearly.