When I started watching Doctor Who*, one of the first things that occurred to me is that the Doctor pretty much always dresses in the same outfit. It seems that after regeneration, the new Doctor finds the right look for him and more or less sticks with it. It feels like it’s tied to identity development, a way from separating the old from the new. The Tenth Doctor did switch between blue and brown suits, but they were the same style. I don’t know if the Doctor just has endless copies of his outfit or if there is a laundry room somewhere in the TARDIS. I’m going to pretend it’s the latter; the thought of a Time Lord washing pants is funny to me.
The Doctor’s traveling companion doesn’t treat clothes in the same way. They wear whatever they feel like, though on one occasion the Doctor provided Rose Tyler with a period appropriate dress. Typically, wherever he and his travel companion go in time or space the locals often accept their wardrobes without comment. I know that being a companion is not about being fashionable, but if the TARDIS had a giant closet, I would be digging through it regularly. Some of the companions have recognizable wardrobe pieces. Rose Tyler wore hoodies in her first season (but then, it felt like every human wore them). Martha Jones wore a leather jacket often enough that AbbyShot sells a replica. Donna Noble also had a fantastic coat. It must have been stashed in one of the suitcases she hauled into the TARDIS. Amy Pond favors short skirts and scarfs.
Even with the consistencies, their outfits change episode by episode in contrast to the Doctor’s look remaining the same. It’s a small touch to differentiate the human from the Time Lord and an effective one.
*Because someone always asks, I started watching on the Ninth Doctor and I’ve only seen the new series so far.