Despite there being an infinite amount of notebooks (meaning an infinite amount of isekaiers), it's a little hard to say but... screw it. It might have to be D in THIS particular notebook :v
(Might have to click on these images to expand)
He may not have super powers, he may not have money, but he's got heart. He's probably the most anime character despite not being an isekaier.
..... And he'll TOTALLY die to whoever you guys paste below. Dude's the Mumen Rider of this franchise. He's probably worth keeping around when you need a distraction. That's a little dark, but I think D's accepted this role.