AN: Spoiler-free. I don't own the Doctor or his companions (or Hi-Fi, for that matter).
On a small corner of a London intersection was a blue police box. Inside that blue police box were the workings of a time machine, known as a TARDIS. Within the doors of the TARDIS was a large, elaborate console room, and within a corner of that very same console room was a small chair with a panda on it.
It wasn't a real panda, of course, but rather a small stuffed animal, which went by the name of Hi-Fi, or, at the very least, was referred to as such by its owner, Steven. The Doctor didn't much care to move it from its place; indeed, he appreciated its company. He didn't much like this travelling alone business, and, at the very least, Hi-Fi could give him some semblance of company. Quite fortunately for the Doctor, that small panda wasn't the only reminder of friends gone by.
In fact, the Doctor had quite a few spare things left about the TARDIS. The equation Adric had worked out to get back to E-Space. A pouch of Janis thorns from Leela. A few cans of Nitro-9 which Ace had concocted herself. An entire array of perfume bottles used by Tegan. Not to mention the hordes of clothing left over from nearly everyone who had boarded the ship at one time or another. When people left, as they inevitably did, they'd never disappear completely. Little remnants would stay behind, reminding him of the impact they'd left on his life.
So the Doctor told himself as he searched for reminders of Amy and Rory. For the moment, however, he contented himself to sit with Hi-Fi, knowing that no one companion could ever be completely erased from his life.
And thank goodness for that.