Jasper and Stewart

Jasper and Stewart are a pair of twin bats that have taken up long-term residence within the TARDIS. They have lived in the TARDIS for "a long time," and The Doctor regards them as old friends. Jasper is the more adventurous and approachable of the two, while Stewart is notably stoic and dutiful.

Though the bats have the run of the ship, they appear to have established a permanent residence in a seldom-used annex of the TARDIS's enormous library wing, near the "H"s.

Biography
Curious about a new presence in the TARDIS's library annex, Jasper landed on the back of a bookcase and crept along the shelf, trying to catch a glimpse of Carolyn McConnel between the books. When Catolyn pulled out the hematology book directly in front of Jasper and saw him she screamed (an understandable response, she was researching Vampires) which caused Jasper to panic and take off.

There was much screaming and flapping before the Doctor arrived and plucked Jasper from the air to give him chin-scratchies and pettings with baby-talking to calm him down. He then formally introduced Jasper to Carolyn and released him back to the rafters.

Stewart watched the whole affair from a nearbye girder with a barely-concealed sense of dry amusement. How silly! (EDA: Vampire Science)

Later when Sam Jones brought The Doctor back to the TARDIS after being imprisoned and starved by the Tractites, he collapsed and slept for 28 hours. The bats helped watched over him; perching on his recovery bed or cuddling in Sam's lap, and when Sam herself had to sleep Jasper accompanied her while Stewart remained to stand guard over The Doctor.

Much later when an empty TARDIS summoned Sam Jones back to it, Jasper and Stewart flew into the console room to greet her and perched on her shoulders for snuggle-times. (EDA: Seeing I)