This article contains plot details that reveal secrets which could result in the reader being rejected.
Sada Kiyoshi, commonly known as Sadakiyo, was a very shy boy who grew up in the same town as Kenji. This shyness was displayed by the fact that he normally wore a National Kid mask (and occasionally a Ninja Hattori-kun mask) at all times.
Sadakiyo wanted to be accepted by a group of friends, most especially Kenji's, but his own self-imposed personal isolation made it difficult for other children to connect with him. Because of his oddities, he also got bullied nearly every day and spent a good deal of time on the roof of his school hoping to communicate with aliens whom he wished would come and take him away.
One day, he snuck into Kenji's Secret Headquarters to figure out what was going on in there, and accidentally stumbled upon Fukubei reading The Prophecy Book. Both boys, seeking companionship for different reasons, decide to become friends. However, Fukubei clearly takes the dominant role in the relationship, telling Sadakiyo that he must call Fukubei "Friend", instead of by his real name or surname. This incident seemed to be the origin of the name Friend, as well. Sadakiyo was accepted into Fukubei's group and later witnessed the Friend's Birth in the school Science Lab along with Yamane and Donkey.
In an incident during her school days recalled by Kiriko Endo, she happened to see Fukubei's group walk by her as she sat reading a microbiology textbook. When Sadakiyo approached her to ask her about the book, she glances over to Fukubei's group and notices that there is a second boy wearing the Ultraman-like mask playing with Fukubei's group.
This "second Sadakiyo" was ultimately revealed to be Katsumata.
In 2002, Sadakiyo was approached by Mon-chan, who had managed to uncover details of the The New Prophecy Book and wanted to question Sadakiyo about the origin of its contents. Sadakiyo, still loyal to The Friend at this point, decided Mon-chan knows too much, and rejects Mon-chan to protect his Friend.
In 2014, Sadakiyo becomes an English teacher in the high school attended by Kanna Endō and Kyoko Koitsumi. On a rainy night, he gives a ride to Kyoko, but takes her to his 'home' instead. The 'home' ends up being the Friend Museum of which Sadakiyo was manager. During his uncomfortable conversation with Kyoko, where he confesses his life story, she manages to get him to realize The Friend may not ever have really been his friend at all.
Sadakiyo decided that he wanted to be known as a good person and betrayed The Friend by setting fire to the museum and helping Kyoko to escape Takasu and her Dream Navigators. He reunites with his fifth grade homeroom teacher who hands Sadakiyo a picture of his younger self that the teacher has taken care of, to which Sadakiyo was greatly touched. Later, he reveals that he killed Mon-chan and hands Mon-chan's notes over to Koitsumi, then he distracts the Dream Navigators by threatening to burn some of Friend's favorite items so the others can escape. As Sadakiyo enters his car and drives away, Takasu, however, lets him go. It is later revealed that his car was rigged to burst into flames in order to kill him.
In the 3rd Year of the Friendship Era, Sadakiyo is revealed to be alive nearby on the outside of the Tokyo Wall as "The Mask King". He became a caretaker for a group of children who had lost their parents to the virus outbreak in 2015. All of these children wear Ultraman masks, just like Sadakiyo.
He apparently also witnessed Otcho climbing the Tokyo Wall. Sadakiyo remarks that, while he knew of those trying to get out of Tokyo, this was the first time he'd ever seen anyone climbing the wall to get in.
Later, Maruo, Keroyon and his son come to the wall in an attempt to get inside Tokyo to distribute a vaccine created by Kiriko Endo to counteract the evolved virus. When the three are greeted with hostility by the Earth Defense Force still policing the gate, they are forced to hide under a sewer overpass where they encounter Sadakiyo's mask-wearing children. The children bring them to Sadakiyo, now known as "The Mask King", who helps the trio by showing them the pegs Otcho used to climb the wall. Maruo recognizes Sadakiyo and asks him to join the struggle against the second Friend.
During Kenji's confrontation with the second Friend, Sadakiyo grappled the Friend and held a knife to the Friend's throat. During the struggle, Tamura Masao flying above the site in a helicopter, crashed it into the last remaining flying saucer and the flaming debris rained down upon Sadakiyo and The Friend.
Sadakiyo is then taken to the hospital where he succumbed to his injuries, but not before telling Kanna that the switch that activates the Anti-Proton bomb lies in the fake Secret Lair in the Expo Theme Park.
Sadakiyo dies peacefully in his sleep when Kenji makes his younger self say goodbye to Sadakiyo (who was moving) inside of the virtual attraction.