Dashell is one of the optional Pixls that can be earned by sucsessfully completing the Flipside Pit of 100 Trials. You need to beat Wracktail to get him.

How do you get piccolo?

The player must take the key to the first floor of Flopside and look for the abandoned house with the locked door near the card shop. The player must use the key to open a door. The player must open the big chest inside to get Piccolo on their team.

Where is dashell Super Paper Mario?

He is unlocked after defeating Wracktail in the Flipside Pit of 100 Trials, and his Catch Card is found on the tenth floor of the Flopside Pit of 100 Trials.