Bowser

Bowser is the primary antagonist of Super Luigi Country and is the game's fourth boss. He is also the main antagonist of the Super Mario series.

Story
'WARNING! Spoilers for Super Luigi Country will be found! Read with caution.'

For whatever reason, Bowser want's to eat Princess Peach, so in order to do that, he kidnaps her. Bowser sends out his finest minions in order to stop Luigi.

Once you finally reach Bowser, it turns out he isn't your finale enemy, Dixie Kong is. It is unknown why and is never explained in-game.

Boss Fight
You reach Bowser after beating Levels 16-20.

When you fight Bowser, his only attack is spitting lines of fire at Luigi, in order to defeat him, you have to touch the axe right behind him. Much like the fight with Captain Olimar, Bowser doesn't move at all.

Trivia

 * Bowser and Dixie Kong are the only bosses in the game that don't get special passwords from Cranky Kong.