They are voice-banks that sound well with each other when used in duets and were made with the ohter in mind.
For example:
MEIKO and KAITO
Rin and Len Kagamine
If one Vocaloid is said to be a counterpart of another, then this is what they mean.
No, she had no male counterpart.
A counterpart is an opposite number, or an equivalent. She was his counterpart on the opposing team.
Her Roman counterpart would be Juno.
Rhea is a Greek goddess and so has no Greek counterpart. Ops/Opis was the Roman counterpart.
Venus is Aphrodite's Roman counterpart.
The male counterpart of a nun is a monk.
If it is M. then that is Monsieur; therefore, the counterpart is Mlle.
The female counterpart of patriarch is 'matriarch'.
I agree with your assessment, but my counterpart doesn't.
The intrusive counterpart of rhyolite is granite.
The female counterpart of a stag is a doe.
Athena's Roman counterpart is Minerva.