Griffin Castillo arrived in Pine Valley at just the right time. He was there to help Amanda Dillon with some car trouble -- and then later the supposed mechanic was able to help Opal Cortlandt through a cardiac episode. For his troubles, Griffin received a "thank you" from Jake Martin in the form of a punch square to his jaw. It was later revealed that while Jake was working for Doctors Without Borders in Africa, Griffin had stolen Jake's wife, Cara Finn, away from him. Griffin didn't earn any bonus points when he announced that David Hayward had taught him everything he knew about cardiology.
Zach Slater had recruited Griffin to assist him with a project for the Miranda Center. Griffin was unaware that Zach had died recently in a plane crash, but Griffin remained in town and was hired to work at Pine Valley Hospital, much to Jake's dismay. Cara arrived in town soon after, and Jake's brother, Tad Martin, investigated Cara. Tad was concerned Cara's presence could impact Jake's marriage, especially after Tad discovered Cara and Griffin had never been lovers because they were siblings. Griffin had pretended to be Cara's lover so she could push Jake away and provide Jake with a reason to divorce her.
Griffin diagnosed Zach's widow Kendall Slater with pneumonia. Kendall was a heart transplant recipient, which made her condition more serious. Griffin arranged for Cara, who was also a doctor, to temporarily move in with Kendall to monitor Kendall's health while she grieved her husband. Griffin often made visits to Kendall's home and office to check on her as well. His expertise in cardiology was put to the test when he performed life-saving surgery on Kendall in the back of an ambulance after she had suffered an aortic aneurysm. Griffin monitored Kendall closely as she recovered, and he agreed to continue the Miranda Center project he and Zach had discussed.
Kendall and Griffin spent a great deal of time together, working on the Miranda Center project, and Kendall stood by Griffin when he was accused of stealing drugs from the hospital and selling them to local teens. Griffin swore he was innocent, but later admitted to Kendall that he had actually stolen antibiotics and shipped them to third world countries. Griffin was suspicious of the other man Kendall was spending time with as well. The man was Ricky Torres, a pastor. Unbeknownst to Griffin at the time, Ricky had become obsessed with Kendall and planted evidence to steer Kendall away from Griffin.
Ricky's plan to make Kendall doubt Griffin did not work. Kendall purchased a date with Griffin through the hospital charity bachelor auction. Soon after, Griffin was accused of the murder of a woman named Diana and was sent to prison. While in prison, Griffin was stabbed by someone working for Ricky. Kendall and her friend Ryan Lavery began to suspect Ricky had been involved with Zach's death. Kendall, Ryan, and Cara helped Griffin escape from the hospital where he was recovering, and Griffin pretended to have left the country. Griffin actually remained in Pine Valley to find evidence to incriminate Ricky. When Kendall and Griffin obtained the evidence that proved Ricky was behind Zach's death, they also proved Ricky had murdered Diana.
Only the stolen medication charges against Griffin remained, and he was able to pay restitution and complete community service. Kendall was thrilled Griffin would be staying in Pine Valley. Griffin admitted to his sister that he would be interested in pursuing a relationship with Kendall if the timing were different. Griffin made plans to head to Chile to oversee the building of a Miranda Center there and he said goodbye to Kendall. She asked him to stay and he admitted that he would be interested in pursuing a relationship with her if the timing were different.
Griffin changed his mind and decided to stay in Pine Valley. Cara and Kendall were thrilled with his decision and he told Kendall he stayed in town because of her. Ryan Lavery was suspicious of Griffin's reason for remaining in town and he suspected that Griffin was somehow connected to David Hayward. Griffin had a key card labeled "Project Orpheus" which matched one that David had. Ryan questioned Griffin but he denied any association with David but Kendall sensed that Griffin was hiding something from her.
The Project Orpheus secret Griffin and David shared was experimental medical research that David had developed. Griffin had assisted David but he was unaware that David had actually tested his experimental procedures on humans. Griffin was shocked when David bragged that he had saved Maria Santos and Greenlee from certain death and also had Dixie and others locked away in a compound receiving treatments. He ended his association with David and shared what he knew about Project Orpheus with Ryan.
Griffin also spoke to Kendall about Project Orpheus and suggested that her husband Zach could still be alive. Kendall refused to think about the possibility and she and Griffin moved forward in their relationship. It ended soon after when Zach was revealed to be alive. Griffin treated Zach at the hospital and monitored Kendall's heart as she dealt with the shock of seeing her husband again. Zach noticed that Griffin's concern for Kendall went beyond a typical doctor-patient relationship and he questioned Griffin about his feelings. Griffin assured Zach that Kendall loved her husband and family and he was only a friend.
Griffin turned his attention from Kendall to David after Zach returned. He felt his eyes had been opened to what others had warned him about in regards to David and he believed David enjoyed using his medical miracles to control the lives of others. Griffin decided to share everything he knew about David's experimental procedures with the FDA. This placed him in direct opposition to both David and Zach since Zach pledged his full support to David and became his partner.
David replaced Griffin as his research assistant and hired Cara which further motivated Griffin to shut down the project. He did not want his sister involved with David in any way. Griffin went to extreme measures to stop David when he allowed David to believe that Zach had kidnapped him to prevent him from speaking to the FDA. He secretly teamed up with Zach, Ryan, and Greenlee to bring David down. Zach pretended he had killed Griffin and then he pretended to shoot Ryan. David confessed he should have let Zach die when the other casino partners planned his murder. The police arrived and took David into custody and Griffin visited his sister.
Cara was surprised by the news of David's arrest but the real surprise was what she shared with Griffin. She informed him she was pregnant with David's child. Griffin begged his sister not to tell David about the child and he promised he would help raise his niece or nephew. He disagreed with Cara's decision to allow David to be a part of her child's life.
David asked Griffin to visit him in jail and he revealed the name and location of two other patients. David explained that no one else had the expertise to care for the patients except for Griffin and he agreed to take over their care. One of the patients was Adam Chandler's twin brother Stuart and he was in critical condition. Griffin transferred Stuart to the hospital and he was reunited with his family. The family was overjoyed when Stuart made a full recovery and planned a party to celebrate his homecoming.
Griffin attended the party for Stuart at the Chandler mansion and he saw Zach and Kendall there. Zach and Griffin engaged in polite conversation and Griffin was present at the party when a shot rang out.
Time advanced five years between the final episode on ABC and the first episode of Prospect Park's online version.
Five years later, Griffin remained in Pine Valley and continued to work at the hospital. He became his sister's confidante after she gave birth to David's child but told David that she had suffered a miscarriage. While David was incarcerated for shooting J.R. Chandler, Cara had used the guise of rejoining Doctors Without Borders to give birth out of the country. Cara left her son Oliver in Puerto Rico with her mother. Griffin and Cara believed that the secret was safe until David was unexpectedly released prison and suspected that Cara had hidden something about her pregnancy.