The best replacement for bowser is dark bowser.

The best alternatives to bowser are rated as following:
★★★★★  dark bowser
★★★★★  mario
★★★  king koopa
★★★  bowser
★★★  koopa
★★  eggman
★★  bowser jr
★★  donkey kong
★★  wario
★★  midbus
★★  peach
★★  dry bowser
★  ganon
★  king k rool
★  giga bowser
★  dk
★  fawful
★  king dedede
★  luigi
  pam ward
  catania
  mario games
  blastoise
  king boo
  robotnik
  yoshi
  mario game
  mario videos
  wart

Your alternative for bowser is not listed? Suggest!
Speaking about bowser, one probably shouldn't forget about peach.

Other search terms related to bowser and their ratings are:
★★★★★ peach
★★★ sha na na
You have in mind a companion for bowser and it is not yet here? Propose!



Google Trends

Scale is based on the average worldwide traffic of bowser in all years. Click on graph for more.
bowser
dark bowser
mario