The Doctor's Litwick
The Doctor ended up with a ghost type pokemon called a Litwick. Litwicks are known to act like a will-o'-the-wisp or ghost lights, leading weary people astray and even to their death. Also it feels just slightly off life energy, which the Doctor will merely shrug off as though it's a nasty habit.
Naturally, the Doctor finds this little Pokemon extremely useful and hopes it will be naturally inclined to lead the Doctor and any interested companions exactly to the places she should not go. This is the Doctor after all.
No plans to evolve this one. It's pretty useful as a little floating candle.