Kaoru Ichijo
The police inspector who is leading operative of the Science Police force formed for fighting the Grongi. Like his father, who died on his tenth birthday, Ichijo is a model policeman who firmly believes in doing the right thing. Though he is very strict with himself, he will do everything he can, even bend police rules, to help Yuusuke fight the Grongi. After the Grongi were defeated, Ichijo returns to Nagano.
Sakurako Sawatari
A friend of Godai who translates the ancient "Lintonese" transcripts that tell of Kuuga and the Grongi. At first she was against helping because she was worried about Yuusuke, but now she believes in him fully and works hard to help Yuusuke understand his powers by deciphering the writings.
Minori Godai
Yuusuke's younger sister who works a teacher at a local daycare. Yuusuke will occasionally visit her there to entertain the kids. Like Sakurako, she was at first apprehensive about her brother transforming into Kuuga, but she believes in Yuusuke and helps out however she can.
Shuichi Tsubaki
A trusted friend of Ichijo, he is the doctor that looks after Yuusuke and treats him when he becomes injured, while also researching the effects of Yuusuke's fusion with the Arcle. Though he's a light-hearted person, he was very serious and worried when talking about Yuusuke's future transformations because of the constant dangers that surrounded the Kuuga legend. He also performs the autopsies of the Grongi victims in order to determine causes of death.
Hikari Enokida
Another old friend of Ichijo, she is in charge of the Science Division of the Police Department. She studies the biology of the Grongi in order to develop effective weapons against them. She also assists in the study of Kuuga and the Gouram. Because of the massive amount of work required of her to fight the Grongi, she was away from home quite often and her child felt alone and neglected.
Tamasaburo Kazari
He was friends with the father of Yuusuke and Minori, who affectionately refer to him as Oyassan like everyone else. He owns and runs the Pore-Pore Tea Cafe. He is a goofy, light-hearted character that keeps a scrap book of all of Kuuga's exploits from the newspaper, though unaware of Yuusuke being "Number 4" and thinks that the name "Kuuga" is some nonsensical word made up by Yuusuke. After learning the truth, however, he dons the Kuuga mark on his apron in the series epilogue.
Nana Asahina
Tamasaburo's niece, who helps out at the Pore-Pore Tea while aspringing her dream to be an actress. However, since the death of her acting teacher at the hands of Grongi, Nana has been troubled until Yuusuke finally helps her overcome her sadness.
Jean Michel Sorrel
A university student from America that shares the work office with Sakurako. He researched the broken fragments that later formed into Gouram. He has a crush on Enokida and hoped to help her reconnect with her child.
Sadao Matsukura
The head of the security department of the Metropolitan Police Department with the head of the unidentified life forms joint investigation headquarters.
Morimichi Sugita
A detective of the investigation dept. 1 of the Metropolitan Police Department.
Tsuyoshi Sakurai
A detective of the investigation dept. 1 of the Metropolitan Police Department.
Nozomi Sasayama
A policewoman of the unidentified life forms joint investigation headquarters who sends communication from headquarters.
Mika Natsume
The daughter of Prof. Natsume, one of Daguva's victims, she was greatly depressed when her father's murder was being overlooked for the Grongi attacks, taking the findings her father had at their house. But after Yuusuke reasons with her, Mika decides to aid in the Grongi matter as Jean's assistant. However, she is unable to follow and decides to take up playing the flute her father gave her. But while attending the 12th Arikawa Group Flute Competition, a hostage situation by a disgruntled government working traumatizes Mika as Yuusuke tells her to not to run from her fear and continue. Three months later, Mika is attending a special college.
Shoji Kanzaki
Yuusuke's elementary school teacher, he served as an inspiration to Yuusuke, and the two of them visit on certain occasions.
Junichi Chono
A young troubled man, he saw the Grongi as a salvation until he was saved by Kuuga. He later researched trying to make a living until he was knocked out by Gamego while on his way to a job interview.
Series:  Kamen Rider Kuuga