English 'her' and 'she' come from different etymological sources — 'her' continues Old English 'hire' (from the Proto-Germanic feminine demonstrative), while 'she' replaced the original nominative 'heo' centuries later, making them etymological strangers sharing one pronoun paradigm.
Used as the object of a verb or preposition to refer to a female person; belonging to a female person.
From Old English 'hire' (her, to her, of her — dative and genitive of 'hēo'), from Proto-Germanic *hezōi (to her, of her), the oblique feminine form of the demonstrative *hiz/*hī-. Unlike 'she' (which was replaced by a demonstrative), 'her' continues directly from Old English 'hire' — so 'she' and 'her' come from DIFFERENT etymological sources. 'Her' predates 'she' by several centuries and survived while the original nominative 'heo' was replaced. Key roots: *ḱe- / *ḱi- (Proto-Indo-European: "this, here (demonstrative)").
'She' and 'her' come from DIFFERENT words. 'Her' continues directly from Old English 'hire' (of/to her), which descends from the Proto-Germanic feminine demonstrative. But 'she' replaced the original Old English 'heo' with a different demonstrative 'seo' in the 12th century. So 'she' and 'her' are etymological strangers forced into the same paradigm — like a married couple with different surnames who share one mailbox.