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Blücher was a dog owned by Sebastian Vaughan. Sebastian named him due to his love of history and took him to Cardiff when his father arranged for him to join Torchwood Three. Blücher would never go outside during bad weather.
After Sebastian's death, Toshiko Sato returned Sebastian's body to Rockfell Hall. She went with Blücher, who was later killed by Sebastian's corpse which had been infected by a cortex leech. Tosh used it to prove to Madeline Vaughan that Sebastian was gone, as he would never have killed his beloved dog. (AUDIO: The Vigil)