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. 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 the beautiful Queen, 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.
Frederic will appear in the upcoming TV series alongside his wife, and will be voiced by Clancy Brown.
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.
The King in Once Upon a Time
The celebration of Rapunzel's birth
Rapunzel's journal sketch of King Frederic, following their emotional falling out