Santiago was a vampire and a member of the Volturi guard.
Santiago was first seen in Breaking Dawn where he arrived to Forks to exterminate the Cullens. Santiago played a small role when he waved goodbye to Renesmee. It shows that he really likes her, but doesn't want to get involved in this situation.
The Short Life of Damien
After returning to to Volterra, Santiago and the male guards were ordered by Aro to find beautiful women, and give birth to half-breeds. Santiago agreed, and went on the hunt. Everything changed until Santiago took his eyes on a beautiful woman named Celia. Santiago revealed what he was, and decided to run away with her, and start a new life. He was caught when one of the guards was eavsdropping on him. Santiago stopped him, and no choice but to kill him. He realizes that he wasn't the only one, and two of the guards came down, but Santiago immediately killed them.
After killing his own guards, Santiago didn't want to serve the Volturi anymore, and wanted to be Isla. Isla wanted to leave Italy with him, and move somewhere far away. Santiago agreed, and later slept with her. This was when she ended up pregnant. Santiago didn't know what to do since he was the only one who can help her with the pregnancy. Isla told him that she loves him until her child breaks free. Santiago tried to turn her, but it was too late since he was holding his own child by his arms. Santiago was devastated, and returned to the underground to show the child to the lords. His son grew up very quick, and ended stopped at the age of 24. Santiago spent a moment with his son, and tells him not to hunt innocent people. Santiago finally revealed to Aro that he killed three of the guards since he was tired of serving the Volturi. Aro was very angry, and he sentenced him to be executed. After Santiago was executed, Aro decided to spare Roosus since he didn't want to waste anytime to kill a created half-breed.
Santiago is described to very beautiful. He is thin, and ranges to 6 feet. His hair is black, and his eyes are crimson when he drinks human blood.
Santiago is meant to be very positive, kind, and a well-known guard. When he drinks human blood from a human, he prays to any body that he killed since he was regretful. When he discovered that Carlisle drank animal blood, Santiago was interested, but he feared that the Volturi would take terrible advantage on him.