
die Gefährtin
companion
A female person who accompanies someone on a path, during an undertaking, or through a stage of life.
Sie war ihm auf all seinen Reisen eine treue Gefährtin.
She was a loyal companion to him on all his journeys.
companion / partner
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companion
A female person who accompanies someone on a path, during an undertaking, or through a stage of life.
Sie war ihm auf all seinen Reisen eine treue Gefährtin.
She was a loyal companion to him on all his journeys.

partner
A female partner in a committed, usually long-term romantic relationship; a life partner.
Nach zwanzig gemeinsamen Jahren nannte er sie nicht mehr nur seine Freundin, sondern seine Gefährtin.
After twenty years together, he no longer called her just his girlfriend, but his partner.
The word belongs to the word family of 'fahren' (to drive/travel). A 'Gefährtin' (female companion) was originally someone who shares a 'Fahrt' (journey/trip) or a path with you. This idea of a shared path is present in both meanings.
Sie war seine Gefährtin auf der langen Wanderung durch die Alpen.
She was his companion on the long hike through the Alps.
'Gefährtin' (female companion/partner) is a rather elevated and sometimes slightly poetic word. It expresses a deep and lasting connection.
In seiner Rede dankte er seiner Gefährtin für die jahrelange Unterstützung.
In his speech, he thanked his partner for years of support.
When a companion for a specific activity is meant, this is often made clear by a preposition, e.g. 'Gefährtin auf seinen Reisen' (companion on his travels) or 'Gefährtin im Abenteuer' (companion in adventure).
Die Hündin war dem alten Mann eine treue Gefährtin bei seinen Spaziergängen.
The dog was a loyal companion to the old man on his walks.
In the meaning 'life partner', the adjective 'treu' (loyal/faithful) is often used ('eine treue Gefährtin' — a loyal partner). A very common and clear form is also the compound word 'Lebensgefährtin' (female life partner).
Er stellte sie allen als seine Lebensgefährtin vor.
He introduced her to everyone as his life partner.
girl
Both words can refer to a female partner. Gefährtin emphasizes a committed, usually long-term relationship and the life shared by the couple. Mädchen is rare and old-fashioned in this sense; it emphasizes the speaker’s personal perspective more strongly and is more likely to appear in older texts.

Nach vielen gemeinsamen Jahren bezeichnet er Anna als seine Gefährtin.
After many years together, he refers to Anna as his partner.
In einem alten Brief bezeichnet er Anna liebevoll als sein Mädchen.
In an old letter, he affectionately refers to Anna as his girl.