Why does ishtar implicate tosaka rin, shouldn't they be treated as two separate entities for tagging purposes (cosplay etc)?
It's because FGO's Ishtar is technically possessing Rin's body (more accurately, both Ishtar and Rin have been merged into a single entity that is a bit more Ishtar than Rin, mentally speaking).
I meant more for tagging purposes; precure-esque shows don't implicate transformed with normal form afaik
Zhuge Liang I can understand since nothing changes, he keeps his old clothes and just gets a cape, but Ishtar gets a completely new wardrobe
It's the same thing with the Dangerous beast cosplay or any other cosplay. It might be Ishtar but it's really more Rin than Ishtar cause all it is is just Rin in different clothing appearence-wise. They're still cosplaying as Rin as Ishtar.
It's also the same as how we tag Caster Cat and Tamamo, how we would tag Castko whenever we have Tamamo despite them different but same entity.
There's also the fact at some point it was sorta established that derivatives would imply their originals. Presumably way back when Saber Alter became an implication to Saber. Or maybe when Saber Bride became a tag that implies Saber Extra. Dunno, it's ancient history by now.