According to Fred, Mr. Frederickson gave him the family mansion while he and Mrs. Frederickson were away on a vacation to the family isles. In the post-credits, Fred discovers a secret lair behind a painting of his family. Mr. Frederikson then appears to his son's amazement and reveals how he has once lived a double life as a superhero. Out of joy and relief of reuniting with him, Fred hugs his father, who hugs him back and tells him that they have a lot to talk about to each other.
Mr. Frederickson first appeared in the episode "Fred's Bro-Tillion". During a trip out of town to deal with an avalanche on the family mountain, Fred is pursued by his father's longtime adversary, Baron Von Steamer, who sought to exact revenge against Frederickson by eliminating his only son, only to kidnap Wasabi by mistake. Frederickson is contacted by Big Hero 6 about the ordeal via his secret headquarters in his mansion. While offhandedly knocking out a yeti, he tells them how to find the locations of Steam's known hideouts. The team will sometimes visit the lair to find a way to eliminate a new foe.
In "Kentucky Kaiju", Frederickson appears on Fred's TV, expecting to speak with his son only to find Hiro instead. While offhandedly battling mummies, he takes note of the Nanovex weave Hiro was using to give himself super strength. Frederickson reminisced about attempting the same thing when he was a rookie hero too before eventually realizing his true strength was his can-do attitude as well as his wealth. He then advised Hiro to leave the premise, as he didn't recognize him at first, so he alerted security.
Since the release of 20th Century Fox's X-Men in 2000, Lee has traditionally made cameo appearances in films based on Marvel Comics properties. Big Hero 6 marks his 22nd appearance in a Marvel-based film. As of March 2016, he has appeared in 24 films based on Marvel Comics.
Him being a superhero is a reference to his past role as the Mayor of Super Hero City in The Super Hero Squad, where he fancied himself a superhero. It could also be a reference to his role as the voice of the future version of Tony Stark in Avengers Assemble.