They're the illustrator's own impressions, but he doesn't say for what exactly... 🤔 A mystery, it is!

I have no goddamn clue what (Iron Will) is supposed to imply here. A quick trip through Twitter tells me it's certainly a slang to put at the end though. If I were to hazard a guess, I'd say it's like "filled with determination" from Undertale. So in this context... probably (Cope).

And yes, it's the "Tamatsukuri" surname for Misumaru.

Additional posts – (1) (2) (3)