create alias nanashina -> isegawa_yasutaka
They are being published (in terms of light novel illustrations) under this name and it is the name at the top of their pixiv and twitter profiles etc.
Isegawa (rather than Isekawa) as that is the way it is spelt out in romanji and/or hiragana by publishers, and also https://www.instagram.com/isegawa_7747/
EDIT: The tag alias nanashina -> isegawa_yasutaka (forum #147182) has been approved by @Hillside_Moose.
Updated by DanbooruBot