Complex sentence: When transformed into Prince Planet an appropriate costume comes with the transformation which resembles the apparel worn by the people of his native world including a helmet, which all male members of Radion society seem to wear.
Split into simpler sentences: When transformed into Prince Planet, an appropriate costume comes with the transformation which resembles the apparel worn by the people of his native world. This includes a helmet, which all male members of Radion society seem to wear.