Varada Sethu's Belinda Chandra and her debut in each version of the Doctor.

Of course, not every Doctor travels with more than one companion. For instance, although Captain Jack Harkness (John Barrowman) was briefly part of the Ninth Doctor's era, Christopher Eccleston's version of the Time Lord only ever truly paired himself off with Billie Piper's Rose Tyler. After Nine, David Tennant's Tenth Doctor traveled with three different companions in consecutive seasons, and Jodie Whittaker's Thirteenth Doctor had an entire cohort at once. So, deciding each Doctor's "true" companion can sometimes be a matter of debate. For Fifteen, I think there's less of a challenge when choosing.

Ruby Sunday Was Great, But Belinda Is The Fifteenth Doctor's Perfect Companion

Belinda's reluctance to TARDIS life hasn't stopped her from proving herself very capable

Don't get me wrong, I thought Millie Gibson did a fantastic job as Ruby. I think the worst thing about her character was completely out of her control, and that was the fact that showrunner Russell T. Davies set her up to have a very exciting origin and then simply revealed her mother was a regular human. However, Belinda's introduction to the Doctor Who season 15 cast in the opener, "The Robot Revolution," proved there was a better fit for Gatwa's Doctor.

While Fifteen often explained in detail what Ruby was seeing for the first time so she could understand, he rarely has to simplify it for Belinda.

While Fifteen often explained in detail what Ruby was seeing for the first time so she could understand, he rarely has to simplify it for Belinda. Sethu's second Doctor Who character - with her first being Mundy Flynn from "Boom" - steps into the story reluctantly, but takes to the adventures far more naturally and with more grace. She comes across like a seasoned pro far quicker than Ruby ever did, and Belinda always seemed to be on the same wavelength as the Doctor during especially trying scenarios.

"Lucky Day" shows just how much Ruby's currently struggling after her TARDIS travels. While she isn't unique in her status as a former companion who is consciously trying to reclaim her normal life, she's one of the most vivid examples of this phenomenon in Doctor Who to date. For Belinda, I don't foresee this being an issue. Sure, Belinda has gradually started to find joy in her adventures with the Doctor, but she ultimately just wants to get home and resume her nursing career. This doesn't necessarily make Belinda a better companion than Ruby, but it is a notable difference.

The Doctor Treats Belinda Like A True Equal

Fifteen gives Belinda far more freedom than some past companions

The Doctor is always protective of his companions, and he always knows more than they do when it comes to how the universe works. That said, he doesn't always treat them all in exactly the same way. With Ruby, it felt like the Doctor was always trying to shield her from the dangers they encountered, whereas he often allows Belinda to stand side-by-side with him when they march into battle. This could have something to do with both companions' respective ages, as Ruby was a teenager when she left with the Doctor, and Belinda was already a grown woman.

Belinda is a more sturdy companion for the Doctor to rely upon, whether he's consciously aware of that or not.

Of course, the Doctor is far older than either Ruby or Belinda, but he'll be aware of the difference in life experience that each character has, and how much they're able to handle. While the Time Lord ultimately grows to trust Ruby with more and more pivotal parts of his plans, Belinda's input is permitted pretty much immediately after they meet, and she even contributes elements of the plans far more often than Ruby ever did. As such, Belinda is a more sturdy companion for the Doctor to rely upon, whether he's consciously aware of that or not.

Fifteen has to work especially hard to earn her respect.

Plus, the Doctor and Ruby always got along. There was never any friction in their relationship. It was wholesome to see, but the lack of personal obstacles they had to overcome made their friendship just a little less rewarding. Belinda is icy with the Doctor when they first meet in "The Robot Revolution," and Fifteen has to work especially hard to earn her respect. As he did so, Belinda gradually brought down her walls. Because of this, I think they have a more substantial bond.

Belinda Is Making A Good Case For Becoming One Of Doctor Who's Best Modern Companions

Varada Sethu's character hit the ground running

There have been some great companions across all eras of Doctor Who. Some stay with the Doctor longer than others, and some have even departed after a single episode. That said, it's not how long they spend as part of the show that decides their legacy, but the quality of the stories in which they're involved and their contributions to them. So far, I've loved every adventure Belinda has been a part of. She hasn't just been along for the ride. Instead, she has been active in her participation and fearless in the face of unpredictable and new situations.

Some previous companions felt like they had been included just to fill the position alongside the Doctor. Neither Ruby nor Belinda fall into that category, but of the two, I think Belinda comes out on top. Not only that, but her efforts have her well on the way to becoming one of the best Doctor Who companions of the modern era - if not all time. At times, she has been the biggest reason the Doctor is even still alive, like when she rescued him from the God of Light in Doctor Who season 15, episode 2, "Lux."

As capable and ready-for-anything as Belinda is, she still thankfully feels like a companion.

As capable and ready-for-anything as Belinda is, she still thankfully feels like a companion. She's the perfect blend of serving as the eyes and ears of the audience, and very prepared to put her newly-learned information into practice without being told a second time. A companion rarely becomes so quickly ready for the dangers that lie ahead, and this trait leaves Ncuti Gatwa's version of the Time Lord free to do what the Doctor has always done and lead the charge in Doctor Who stories with perfectly-suited .