This is a very special Crawfish named Nancy, in case you needed convincing.
These are marked by an orange shopping cart on your map.
However, you won’t find the sushi set in the Poppo nearest the homeless man.

Instead, you’ll need to trek over to the Poppo nearest Jinnai Station.
And that’s pretty much it!
Simply head back to the homeless man and he’ll give you Nancy in exchange for the set.

To give you some context if you’re just a curious visitor passing by.
There’s a substory called “The Crawfish Caper” which becomes available from Chapter 5 onwards.
And this crawfish’s name is Nancy, by the way.

What rewards do you get for handing it in?
This is spoiler territory, so bear this in mind before reading on below.
And they can get you out of some really tricky situations, so they’re worth nabbing.

It only works on humans, deals decent damage, and is likely to poison your chosen targets.