Place of origin:
Santiago was named for Santiago, Chile, where he was born in a caravan at the foothills of the Andes Mountains. He had travelled the world with his parents, protesting for environmental and political causes. However, he had felt estranged. His family was scattered all over the world for months at a time because of their lifestyle. When The Doctor, who he had always wanted to meet, was declared dead, Santiago was taken to the UNIT base. There, he met aliens, including the Shansheeth and Groske, for the first time. He was able to quickly made friends with Rani and Clyde and had witnessed a plot by the Shansheeth and Colonel Tia Karim to steal the Eleventh Doctor’s TARDIS by making a key from the memories of his grandmother and Sarah Jane Smith. Santiago was advised to make some changes in his relationship with his parents. After a momentary trip in the TARDIS, he left with his grandmother to travel to Norway by hovercraft. (Death of the Doctor) There he was going to meet his parents. They wanted to take part in an anti-nuclear rally.(Death of the Doctor)
Behind The Scenes
Santiago is one of the few grandchildren of companions to have been portrayed, along with Gordon Lethbridge-Stewart, Donna Noble, Steven Carter, and (to the extent that Brian Williams is considered to be a de facto companion or “companion presumptive”) River Song. He, River Song, Donna Noble and Susan Foreman are the only known characters whose selves and grandparent have both travelled in the Doctor’s TARDIS; he is the only such character to have travelled with their grandparent in the TARDIS since the First Doctor’s granddaughter, Susan Foreman, in 1965 (2165 in-universe).
There were plans for Santiago to return to the screen in the Sarah Jane Adventures Series 5 story The Battle for Bannerman Road, but the passing of Elisabeth Sladen resulted in the story being scrapped.