I'm a redhead, and I absolutely disagree! There are many redheaded males and females playing good people, and they generally have a great sense of humor, rather wry at times. It's definitely not true about them being adversaries.
Check out the movies of these people: Katharine Hepburn, Van Johnson, Darren McGavin, Greer Garson, Rhonda Fleming, Julianne Moore, Carol Burnett, Bette Midler, Diane Lane (who stayed redheaded for most films after "The Outsiders" in which she was a nice girl), Debi Mazar, Sara Rue, Debra Messing, Bryce Dallas Howard, Kirsten Dunst (as Mary Jane), David Caruso, Ginger Rogers (sometimes), Seth Green, Lindsay Lohan (prior to her major troubles), Nicole Kidman (before she went blondish), Kyle Schmid, Rita Hayworth, Arlene Dahl, Danny Kaye, Red Skelton, Alicia Witt, Laura Prepon (before she went blonde), Molly Ringwald (John Hughes' leading lady in those teen comedies), Gillian Anderson, Marcia Cross, Amy Adams, Isla Fisher, Lucille Ball(!), Maureen O'Hara, Alyson Hannagan, Eric Stoltz, Ron(nie) Howard, Bill(y) Mumy, Clara Bow, Kevin McKid, Gates McFadden ("Star Trek"'s lady doctor), Judy Garland (in a number of movies), Christina Hendricks, Roz Kelly.
(I'm sure more will come to mind.)
Some of these have played some unsavory, baddish roles, but the majority of those roles falls into the good side.
Some examples:
1) The Wizard of Oz ~ Redhead Billie Burke portrays Glinda the Good Witch.
2) The Fifth Element ~ Leeloo is pivotal in saving Earth!
3) The Little Mermaid ~ Ariel has blazing red tresses! See? There's even a redhaired Disney princess!
4) Annie ~ stars a redhaired orphan girl; based on the long-running "Little Annie Rooney" comic strip
5) Elizabeth I ~ There have been many movies and miniseries focusing on this auburn-haired monarch.
6) Seven Brides for Seven Brothers ~ stars seven handsome, redheaded men, the Pontipee brothers
7) Life With Father ~ a familyful of redheads (an absolutely delightful movie filled with great characters!)