In a universe filled with genocidal cyborgs, fanatical religious orders, and killer alien statues, it’s safe to say that the titular Doctors of Doctor Who have faced their fair share of challenges over the years. From sword-fighting Sycorax gangsters in the skies over London to outwitting Cybermen on an alternative version of Earth, each Doctor on Doctor Who has proven themselves capable of standing up to the galaxy’s most significant threats time and time again.

Of course, one large reason the Doctor has emerged victorious from each of his interstellar battles has to do with the support of his loyal companions. Typically hailing from Earth and joining the Doctor on his travels, these companions make all the difference in the Doctor’s universe-spanning adventures, providing a humanistic counterbalance to everyone’s favorite time-traveling alien.

Sarah Jane Smith

The fact that Sarah Jane Smith received her own spin-off show should tell viewers all they need to know about this character’s unrivaled popularity. Originally introduced as a companion for the Third Doctor way back in 1973, Sarah Jane went on to appear opposite five other Doctors over the next several decades. An intrepid investigative journalist capable of holding her own against a Dalek or rampaging Cyberman, Sarah Jane set the gold standard for what Doctor Who companions could and should look like. Rather than appearing as hapless tagalongs waiting to be kidnapped, Sarah Jane broke the mold for the traditional TV sidekick, proving herself every bit as tough, spirited, and intellectually gifted as the Doctor (which is certainly saying something).

Rose Tyler

Few companions have brought out the humanistic qualities of the Doctor quite like Rose Tyler. The first companion of the rebooted Doctor Who series, Rose had some rather large shoes to fill when it came to matching her predecessors. Yet like Christopher Eccleston’s energetic Ninth Doctor, viewers couldn’t help but fall in love with Rose’s character from the moment she first appeared on-screen. A quirky and relatable London shop assistant, Rose’s relationship to the Tenth Doctor shined a light on an underexplored aspect about the Doctor’s personality, hinting at a mutual romantic attraction unseen in most other Doctor Who companions before or since.

Amy Pond

In contrast to the will-they, won’t-they romantic dynamic of the Tenth Doctor’s companions, fans  tend to take note of Amy Pond’s far more platonic relationship to Matt Smith’s Eleventh Doctor. Establishing herself as an iron-willed singing telegram transplanted from Scotland to rural England, Amy provided a source of constant stability and support for her fellow companions in Doctor Who. Counterbalancing the Eleventh Doctor’s childish antics with an almost parental sense of responsibility, Amy’s stern stance on the Doctor’s actions allowed them both to grow into significantly more mature individuals over time. Add in Amy’s husband Rory Williams as a kind-natured if somewhat absent-minded fellow companion, and you have the making for a truly remarkable trio of interplanetary adventurers.

Donna Noble

If Rose appeared as a potential romantic counterpart to the Tenth Doctor, then one might describe Donna Noble’s relationship to the Doctor as that of the bickering little sister. Without a doubt among the most hilarious of the Doctor’s many companions, every time Donna appeared on-screen, it wasn’t long before she was locked into some sort of verbal sparring match with David Tennant’s Converse-clad time traveler. An ordinary temp worker who initially balks at the chance to accompany the Tenth Doctor aboard the TARDIS, Donna’s humorous banter with the Doctor made her a companion viewers were clamoring to see more of following her initial 2006 debut.

Captain Jack Harkness

As with his preceding companion in Sarah Jane Smith, Captain Jack Harkness’s incredible popularity eventually allowed him to star in his own distinct spin-off series: the far grittier and more adult-oriented Torchwood. But before he was leading a covert team of alien-fighting government agents, Jack Harkness captured audiences’ attention with his flirtatious, charismatic personality on Doctor Who. At once charming and roguish in both his personality and mysterious motives, Jack’s repeated appearances in Doctor Who never fails to gauge audiences’ interest, establishing this 51st-century con man as the Whovian equivalent to Han Solo or James Bond.

Clara Oswald

In her earliest appearances on the show, fans couldn’t help but wonder who on earth the enigmatic Clara Oswald really was. Cropping up in different timelines but somehow always managing to meet with the same grim fate, Clara’s presence in Doctor Who formed a taut central mystery that even the Doctor struggled to figure out. Piquing our collective curiosity from the get-go, Clara only continued to solidify herself as a brash and colorful companion upon joining the Eleventh and Twelfth Doctor on their travels. Never afraid to speak her mind, Clara’s unique, somewhat confrontational personality made her an ideal counterpart to the similarly-tempered Doctors she found herself with.