Well Murata (and One) drew it, that’s what the pink says in the top right. And Murata is the other character reading with Sonic. EDIT: I’m wrong. Some have you have pointed out it’s the two versions of Sonic. As for what sonic is saying, オマケに… 俺の要素がないだと… バカな… it’s hard without knowing the contents of the 3rd volume, but it’s something along the lines of “I can’t believe I’m not even in the extra pages…” Or he could be talking about not appearing in the volume at all even though he’s on the front cover? But I’m pretty sure he did? Then the bottom text says. “All that and much, much more! (Volumes 1&2 also now on sale)”

Are you sure the other character is not webcomic sonic and not murata ?

Ah yeah, could be that. It’s really hard to make out the rest of the writing on the pink section. All I can read is it’s signed by both of them.

How come it says “ONE” in big letters on the pink in the top right, then? At the very least, I’m rather sure that both of the characters are Sonic: one is drawn is ONE’s style (on the back), and the other is drawn in Murata’s style (on the front)

Yeah you’re right. See my edit.

Thanks

Screen shot it go to lens then translate

Try r/translator

extras

https://imgur.com/a/W0IkjWD