Young Master of London Arc (Kinnikuman Nisei Manga Chapters 294-300, Manga Only)
This is a bonus story that features in Volume 29 of Kinnikuman Nisei, which tells the story of Robin Mask's relationship with Alisa Mackintosh; although it appears towards the end of the series, it has been placed here for chronological order. 
Robin Mask tries to win over the father of Alisa Mackintosh: Paul Mackintosh. He believes that by becoming human, he will win the approval of Paul, who harbours anti-chojin sentiments. Robin nearly forfeits his position in the British preliminary rounds of the Chojin Olympics, until he sees John Bullman killed in the ring by Guillotine King. This inspires him to fight in the finals, at which point he avenges Jon Bullman and wins the respect of Alisa's father.
We are introduced to Robin "42 Years Ago" from the Demon Seed Arc in Kinnikuman Nisei, where he acts as the rugby captain in Oxford University and scores every point in the match as the most popular player. He is dating the team assistant, Alisa, and he is seen to be very excited about becoming a wrestler and meeting Alisa's parents. He acts unwittingly in an uncouth manner, to which Alisa's father derides him from snobbery and forbids their union.
Robin drowns his sorrows in a bar, where he engages in fights and gets drunk, only for John Bullman to intervene. He then reveals to his father, Robin Knight, that he wishes to forfeit his Chojin status and become human for Alisa's sake, just as she was willing to give up her family for his sake. He is given the blessing of Robin Grande and the "Icon of Robin", that will allow him to become human. Robin goes to Stonehenge and reneges on his powers, thus becoming human.
After seeing John Bullman die in the ring, he flees in order to assume again his position of Chojin to protect the people, and gives up his second chance with Alisa's family to be a Justice Chojin. He is ganged up outside a bar by hooligans and beaten up, only to be rescued by Terryman, who gives him back his powers with the stone used to remove them. He then goes to the ring to defeat Guillotine King. On September 9th, 1976, he wins the battle with the support of Alisa, making his debut.