Bowser

Role
Bowser is a very important character obviously. He is the person who set out, partially planned, and killed many. Bowser attacked the kingdom twice in attempt to capture Peach and succeded. He is responsible for many deaths including his own henchman, Waluigi. Unfortunately enough for him, he lost the first war. But we may see a return to him.

Possible Death...
Died in Season 1 - Part 10 - "The Finale"

He was killed by Mario who incinerated him nearby the canyon. However, Bowsers body was found in S2 E2, and may be resurected as heard amongst the others. So we'll call this a possible death.