In the original film, King Frederic was said to be a beloved ruler alongside Queen Arianna, and was also shown to be deathly devoted to his family. When Rapunzel was kidnapped by the wicked Mother Gothel, he was devastated, and remained as such for the following years to the point where the anniversary of Rapunzel's disappearance drove him to tears even eighteen years later. The moment his daughter was returned, however, King Frederic embraced his restored family and, as seen in Tangled: Before Ever After, vowed to never let anything happen to them again, especially Rapunzel.
Gothel's actions drove Frederic to an understandably paranoid state of mind. Likely suffering from Post Traumatic Stress Disorder, Frederic cracked down on his security forces throughout the kingdom. Despite his overwhelming joy toward Rapunzel's return, he was admittedly unable to separate "the father from the king", therefore creating laws, rules and restrictions that limited Rapunzel's freedom in hopes of preventing another catastrophe similar to his daughter's kidnapping at the hands of Gothel. This causes tension between Frederic and Rapunzel, but despite his firm attitude on the matter, Frederic tries his best to make Rapunzel feel happy and content with her new life.
Father to Rapunzel, and husband to Queen Arianna, Frederic is the ruler of Corona. He is first seen by his wife's sickbed while she is heavily pregnant with their daughter, and lighting the lantern with Arianna later on after Rapunzel's birth. Every year on Rapunzel's birthday, Frederic and Arianna release a lantern for their daughter. He is seen releasing the lantern again, the night Rapunzel arrives with Flynn Rider to watch the lanterns float. He weeps silently while preparing for the launch of the first lantern, and is comforted by Arianna. These scenes show that he and Arianna have a close and loving relationship and that they still grieve for their lost daughter. Frederic returns to the scene towards the end when Rapunzel returns to reunite with him and Arianna. He is then seen again smiling and hugging Rapunzel and Arianna. Although Frederic, like Arianna and many other minor and supporting characters, does not speak, he has vivid and lively facial emotions and body languages that loudly and clearly express his thoughts and ideas.
In the short film, Rapunzel and Eugene's wedding day has finally arrived. Frederic is obviously present the day of, and walks Rapunzel down the aisle. After escorting his daughter to the altar, he sat down to join his wife in watching the ceremony as the priest officiated the wedding vows. Upon asking Maximus and Pascal for the rings, everyone, Frederic included, were shocked to see the two animals covered in tar, and stared awkwardly as they held up the rings to the happy couple. After exchanging rings, the priest officially declared Rapunzel and Eugene married. Frederic hugged his wife while the rest of the audience cheered.
Taking place six months after Tangled and before the events of Tangled: Ever After, Frederic is overjoyed to have Rapunzel in the sanctity of the castle once more. As her coronation draws near, Frederic reminds his daughter that the duties of a princess are important for both the kingdom and their family, despite how time-consuming they can be. Rapunzel asks for some down time, but Frederic believes it would be unnecessary, and that Rapunzel simply needs some time to settle into the lifestyle. Additionally, in preparation for the coronation, Frederic has upped the security in Corona, including a wall blocking the kingdom from the rest of the world. This wall, and her father's overbearing nature as a whole, has Rapunzel feeling trapped and confined. With the help of her handmaiden Cassandra, Rapunzel crosses Corona's wall and journeys into the world surrounding it overnight. During the outing, she finds a mysterious rock that causes her magic golden hair to return.
Back at the castle, Frederic and Arianna talk over the former's paranoia, and this encourages the king to at least rethink his methods of protecting Rapunzel. At the coronation, however, Frederic nearly becomes victim to a revenge plot headed by a pirate named Lady Caine. Rapunzel protects her father and the other royal guests by using her hair as a weapon and fighting the Pirates off. After Caine's arrest, Rapunzel explains the truth to her father about the hair. In response, Frederic tries to explain the reasoning for his overprotective behavior, explaining that, after his daughter's kidnapping eighteen years prior, "a part of him died", and since then, he has been unable to shake off the worry that a new danger is always close by. As such, he feels the return of the magic hair may attract a threat similar to Gothel, and thusly enacts a new law that prevents Rapunzel from leaving the confines of the kingdom without his consent. He also orders his daughter to never speak of magic of any kind from that moment forward, all of which drives her to tears. Frederic, meanwhile, can only hang his head in guilt and disappointment at this, as he secretly feels he is doing a terrible thing to his daughter.
Frederic appears in the series taking place immediately after Tangled: Before Ever After, where he continues to govern the kingdom while also finding time to grow and develop as a person, specifically in his relationship with Rapunzel. In "What the Hair?!", a week after Frederic and Rapunzel's falling out, it is revealed that neither of the two have spoken to one another about the matter, which distracts Frederic a great deal from his normal activities. Arianna urges him to talk with his daughter, but he refuses. In "The Wrath of Ruthless Ruth", Frederic finds Rapunzel's journal, opened, and fondly looks through his daughter's illustrations. He soon finds, however, that the illustrations are recaps of his daughter's recent, high-risk adventures within the kingdom. Fearful that Rapunzel will get into harm's way despite obeying his law of staying within Corona's walls, Frederic orders the Captain of the Guards to double down on Rapunzel's security. Rapunzel quickly takes notice of this and becomes motivated to confront her father on his shielding. Upon arriving within her father's quarters, Rapunzel finds the king in a distraught state. He hurriedly embraces his daughter and professes his love for her. Rapunzel comes to realize that her father's actions are out of genuine love, and in response, chooses to keep her feelings undisclosed by simply returning his sentiments.
Frederic and Arianna make a brief appearance at the end of the episode "The Tower", where they are seen reuniting with Rapunzel after years of separation.
Interestingly, by the end of Tangled: Before Ever After, Frederic's paranoia has morphed him into an overbearing parental figure eerily similar to Mother Gothel, as he forbid Rapunzel the freedom she craves. The difference is Frederic's actions are out of genuine love, while Gothel's were out of selfishness.
In "In Like Flynn", Frederic mentions that he's had extensive military training.
According to Rapunzel in "In Like Flynn", Frederic likes to tell jokes, though they're not very good.
The King in Once Upon a Time
Rapunzel's journal sketch of King Frederic, following their emotional falling out