Deadbeats are cool, because any skeleton shown can be called one. Which often results in hilarity. And our specific design, with coat and headband is pretty sick. Death-Sensei is cute, and I have my own amusing headcanon that it was supposed to be a plain soul orb (Which he monitored his apprentice through) but she made him look cute and put shades on him. The real Death-Sensei didn't know for some time and when he finally saw, he was like "And what is that?" and Mori tried to play it off.

But in terms of actual mascot and fanbases, I think Shiori wins. Yorick is a neat design and all the different classes of Novelite have fantastic merch/lore potential. Along with the growing bestiary of Inkwell.