I wouldn't say there's so much a least important character as there is a group of least important characters. Twenty of them aren't even important enough to be in the game. Eleven of them should have merely been minor background characters. Ten of them should have been major background characters but still not in the party. Only four of the characters really mattered at all. As for who belongs in each category?

Shouldn't even have been there: Turnip, Grobyc, Draggy, Leah, Miki, Skelly, Greco, Starky, Janice, Van, Zappa, Funguy, Niofio, Pip, Mel, Pierre, Guile, Mojo, Orcha, and Poshul.

Should have been minor background characters: Doc, Orlha, Steena, Irenes, Sneff, Norris, Sprigg, Luccia, Razzly, Leena, and Macha.

Should have been major background characters: Marcy, Zoah, Karsh, Riddel, Viper, Fargo, Radius, Glenn, Korcha, and Nikki.

The only characters that actually mattered: Serge, Kid, Lynx, and Harle.