The reason Ereshkigal used Ishtar's body was mainly because Ishtar and Ereshkigal had descended together. Moreover, in an era even further back, Ishtar and Ereshkigal should have been existences with the same divinity.
Both Ishtar and Ereshkigal were so-called Earth Mother Goddesses.
Ishtar was the Earth Mother Goddess who governed bountiful harvests and life.
Ereshkigal was the Earth Mother Goddess who governed withering plants and death.
If Ishtar was the Lady of Heaven who symbolized "the sky" and "life," then Ereshkigal was the Lady of the Underworld who symbolized "the earth" and "death."
These two divinities were two sides of the same coin. Like light and darkness, they were two divinities born after an ancient god had been split in two. After all, the giant god Attila had ravaged the Age of Gods quite severely back then!
In a sense, they were each other's other half.
Therefore, after being summoned, she was Ishtar in the morning and Ereshkigal at night.
"Eh—r-r-really!? I-I-I... do I really have to go?"
Ereshkigal's entire face flushed red. Ishtar, were you so full you had nothing better to do than make promises like that?!
So Ishtar was that kind of person? Taking advantage of wearing a face that was not originally hers to do something like this?
Her body trembling slightly, Tohsaka Rin looked like a great demon king in Ereshkigal's eyes at that moment.
"Go on! You're a goddess, aren't you? Don't tell me you're chickening out at a time like this?"
For some reason, although Tohsaka Rin did not particularly dislike this person wearing her face, she still felt somewhat unwilling to concede. After all, losing to anyone else was the same as losing to herself.
And she had taken her appearance for no apparent reason. It felt like putting the name Ishtar