Della is being nice, Inigo will at least agree on that. Nice enough that she manages to break through his surprise for a moment, just enough for him to blush a little. "Thank you for the offer. It's very kind of you.."

But then. There's Tidus. Opening his mouth again.

The blush is instantly gone.

"Don't hit me with spells?!" He feels like this should be one of the basic rules of brodom! Della! Della, look, do you see what's going on here? Tidus is threatening him! "And also, I do eat and sleep on missions!"

There's a pause, where he seems to feel too bad about pulling out such a bold lie in front of either Tidus or Della, and then he adds:

".. sometimes!"