StarioStario54321 is the Star World counterpart of MarioMario54321. He lives in the Star World and met MarioMario54321 (along with the rest of the group) in Into the Fourth Dimension. Considering he is a counterpart, it can be assumed SS has been through similar adventures as MM.
StarioStario54321 first appeared in this movie when the group came to the Star World to get to the Fourth Dimension. They came when the discussion about counterparts began to be brought up. StarioStario was one of the revealed counterparts. StarioStario was briefly mentioned when MarioMario was mind controlled, and again when Mario used the Mega D-Star and destroyed Dimension D.
StarioStario54321 also made a brief cameo in Blooper 40 - Part 9, where he was cheering Starman3 on to get up.
- Mushroom Kingdom/Blooper Land counterpart: MarioMario54321
- Fourth Dimensional Counterpart: QuarioQuario54321
- Crown Dimensional Counterpart: QuinarioQuinario54321
- Dimension X counterpart (doppelganger): OiramOiram12345