Who: Rose Tyler, a companion of the Ninth and Tenth Doctors in the revival of the British serial Doctor Who:

Why: Doctor Who had just about everything: a time machine, the ability to regenerate, and the staying power to remain on TV for four decades. But before the series revival in 2005, he didn’t have a companion who was more popular than he was. Enter Billie Piper’s Rose Tyler, a 19-year-old London shopgirl. From the moment Christopher Eccleston’s Doctor walked into her life—saying just one word, “Run!”—Rose became the heart of a series that had always revolved around the man in the blue box. Her chippy British slang, waifish looks, and emotional reach allowed her to retain the series’ fans when Eccleston left at the end of the 2005, to be replaced by the charismatic David Tennant. Rose’s impact was so deep that when she left in the 2006 season’s heartrending finale, the Whovian fan base spontaneously combusted.

Impact: Before Doctor Who, Billie Piper was a chirpy blonde pop tart, charting some creampuff UK top 40 singles. Fans of the older series wondered aloud whether she could bring the gravitas necessary for a major character like Rose. (Fans of the older series also think Daleks and Cybermen are cool, but whatever.) As with Michael Keaton in Batman, Piper nailed the character out of the gate. Now she’s a major star, headlining her own series, Secret Diary of a Call Girl. Meantime, she’s back on Doctor Who for a three-parter in which, unsurprisingly, she has to save the universe. Neat.

Personal Connection: For decades, I thought Doctor Who was terrible. With the exception of a short Tom Baker run featuring companions Sarah Jane Smith, Leela, and K-9 (all previous contenders for best-companion-ever), I could never get into it. Now I wouldn’t miss an episode. Maybe Eccleston and the exceptional writing would have hooked me on their own, but Rose sealed the deal. I’m a Whovian. (2005+ only, that is. I still think Daleks and Cybermen are ridiculous.)

Other Contenders: Teller, Penn Gillette’s silent partner in legerdemain; Snoopy’s avian Passepartout, Woodstock; Jerome, Morris Day’s mirror man in the band The Time; Lockheed, the alien dragonet belonging to X-Man Kitty Pride; Ron and Hermione, Harry Potter’s friends till the end; Blackadder’s dogsbody Baldrick, who has a cunning plan which cannot fail.