Gustavo Romero Tercero was born on December 16, 1971, in Valdepeñas, Spain. He grew up in this small town, where he later married a woman named Yolanda Saez. They married young and had two children together. At the time of Gustavo's first murders, Yolanda was pregnant with their second child.

Throughout his early life, Gustavo exhibited troubling behavior. He had a history of sexual obsession and voyeurism. He often went to the public park in Valdepeñas, where he would watch couples without their knowledge. This behavior indicated a lack of respect for others' privacy. He was also known for being violent toward his wife and children, although outsiders did not see him as a dangerous person.

On June 18, 1993, Gustavo Romero Tercero attacked Ángel Ibáñez and his girlfriend, Sara Dotor. He forced them to a dark area next to a railway and threatened Ibáñez with a knife. After taking Ibáñez’s wallet, he stabbed him multiple times, even after he was dead. When Dotor recognized him, Romero chased her down, stabbing her and attacking her before finally killing her. He disposed of evidence by throwing Dotor’s clothes into a river and hiding the knife near his home. Just five days later, Romero left town.

In 1997, Romero returned to Valdepeñas and made friends with Dotor’s brothers. He often asked them about the investigation into their sister's death. On June 25, 1998, after finishing his work at a brothel, Romero encountered 21-year-old Rosana Maroto while she was cycling. He claimed he accidentally hit her with his car. Believing she was dead and fearing exposure for his previous murders, he placed her body in the trunk of his car.

Romero later undressed Maroto at an abandoned farm. When she regained consciousness and demanded her bike back, he ended up strangling her with her shoelaces. He then threw her body into a well and covered it with stones. Maroto’s disappearance attracted media attention. A search for her turned up her belongings, and police discovered blood linked to a male DNA sample. Despite extensive searching, her body was not found until 2003.

In 2003, Romero's wife reported him after an incident of domestic violence. During his time in custody, authorities tied him to the 1993 murders of Ibáñez and Dotor along with Maroto’s disappearance. He confessed to all three murders and led the police to a knife used in the earlier case as well as Maroto’s remains.

On April 22, 2005, Romero was sentenced to over 103 years in prison for his crimes. He received a lengthy sentence for each murder along with additional time for rape and robbery. He was barred from returning to Valdepeñas for a decade and had to compensate the victims' families. Police also looked into the possibility of other crimes he might have committed but did not find enough evidence to charge him with those murders.