Companions were the Doctor's closest friends. Such people knew the Doctor's "secret": that he was someone non-human who travelled in space and time in a police box-shaped craft called the TARDIS. They often directly saved his life, or provided him with a perspective that prevented him from abusing his Time Lord powers. On some occasions, they were the proximate reason that the Doctor sacrificed his then-current life and regenerated. On others, they lost their lives in pursuit of the Doctor's goals.
Because the term was a shortening of the expression "travelling companion", most companions did in fact voyage with him in the TARDIS. Depending on the situation, other words were used to describe the same relationship — most frequently, "assistant". Both terms were occasionally troublesome. Sometimes, the word companion was interpreted sexually, leading to confusion about the Doctor's relationship with his friends. Sometimes, companions bickered over the appropriateness of the word assistant. Rose Tyler said emphatically, "I'm not his assistant", when Sarah Jane called her one. Rose seemed happier being called a "companion", as when Harriet Jones called out over the Sub-Wave Network for "former companions of the Doctor".
We track thousands of characters on this wiki, from the major to those that are only implied to exist through dialogue. To find them, it's awfully useful to understand something about our categories: we don't call 'em characters, but individuals. To start delving into our database about characters, just type category:individuals into the search bar. (Or you could just click that link — but that's kinda cheatin'!)
Individuals by species organises characters by species. So if you're looking for a particular Sontaran or a specific Aridian, this is where to come. (Watch out for those individual Farrashians, though: they're just mean!)
Individuals by location gives you a sense of characters based on where they encountered the Doctor or other major characters. For instance, say you wanted to know other people who lived on the Powell Estate with Rose Tyler. All you have to do is go to Powell Estate residents to hit your search for six!
Let's face it: the DWU is full of crazy names. We try to give you a little note at the top of the page if you're within a letter of another character, but sometimes we haven't discovered the similarity yet. If you're at all in doubt about spelling, it's probably a good idea to actually go to the page and check it out!
Looking for companions? Aren't we all, sister! You could go broad and just look at companions of the Doctor. Or, you could narrow things down by looking for the companion of a specific Doctor — like category:Second Doctor companions.
Here's a li'l tip: all Doctors' categories work the same way: number + Doctor + whatever. So, Fifth Doctor companions, Fifth Doctor comic stories and so on. Try this: enter just the phrase Category:Fifth in the search box on top of this page and see what pops up!
When we say we track obscure characters, we mean it! Look at how much stuff we have at individuals with unknown names!