Waluigi

Role
Waluigi was a very large part of many of the attacks. The first two attacks were coordinated by him and were executed by him. Besides this he basically did Bowsers job for him, leading his minions. So overall, Waluigi was very important to the series.

Death
Died in Season 1 - Episode 6 - "Collapse"

Waluigi's death was as simple as just Bowser being stupid. He betrayed him after the long fights and seeing as he didn't need him, when he really did. He was stabbed at the edge of the command center in the castle, and fell to his death. This arguably was the cause of their defeat.