The following is to cover the tradition for having a girl character in every series. Sometimes they serve as love interests, girl fridays (assistants) or just a friend who just happens to be a girl and is the only girl in the series. Most of the time in the beginning these women serve as the 'damsel in distress.' I am covering only the major ones. I am not that familar with some of te Showa Era series so I might be wrong. There is an unspoken rule that they are not suppose to become Kamen Riders, the only exceptions being Human Tackle and Natsumi becoming Kamen Rider Kivara.
The first 'love interest' girl character in Kamen Rider. The daughter of Prof. Midorikawa, blaming Hongo for his death.
In V3 (1973), a young woman who has a crush on Shirou. He is aware of her crush, but refuses to let her get close to him, to protect her, because he lives such a dangerous life. Despite this, she assists Shiro, often watching the communications station, and eventually becomes a close friend to Shirou.
In X (1974), Keisuke's fiancee, Keitaro's assistant, and a G.O.D. member. She is an Interpol investigator and executes undercover operation in the G.O.D. She became a cyborg by G.O.D.'s operation. Her true identity was exposed protecting a child from Strong Arms Atlas and she was destroyed by the General of G.O.D.
In Amazon (1974), Mashahiko's big sister. At first she doesn't trust Amazon due to bringing Geddon to Japan and causing chaos. After being constantly rescued and helped by Amazon she puts her faith and trust in him.
In Stronger (1975), she is a modified human by Black Satan like Stronger and becomes Human Tackle. She had feelings for Stronger and died for him.
I skipped Skyrider because I am unfamiliar with it and have no info or pics on it.
In Super-1 (1981), Tani Motor Shop employee. I haven't seen the series, so I am not that familiar.
Kyoko Akizuki and Katsumi Kida
I really didn't know which one to cover the most, so I did both. Right is Kyoko is Nobuhiko's younger sister, targeted a number of times by the High Priests as her lifeforce could awaken Shadow Moon. And at Left is Katsumi (Nobuhiko's girlfriend). She assists Kotaro quite a few times to persuade Shadow Moon to remember his human-side. After the defeat of Black (1987) at the hands of Shadow Moon, she and Kyoko left Japan to start a new life abroad.
Reiko was Kotaro's girlfriend in Black RX (1988). She was also a photojournalist, she helped him before and after learning he is RX. After learning that Kotaro is RX, she begins to train herself in order fight Crisis's minions.
In Shin (1992), she is his girlfriend and nurse at the ISS where the experiments are held. She tells Shin of the conspiracy, only later to be killed while pregnant with Shin's child. With her dying breath, she tells him to protect the child at all costs, for it is now a target of the syndicate due to its mutation.
In Kuuga (2000), she is 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.
In Agito (2001), she is the system's monitor engineer and was behind the G-3 system. She was in constant rivalry with Toru Hojo. She is very glad when learning that Shoichi is Agito. In the series epilogue, she left the police force after the Overlord incident and moved to England to work at a university.
In Ryuki (2002), she is a very central character. Initially helping out both Ren and Shinji, she could see the Mirror Monsters even though she wasn't a Kamen Rider and looking for her lost brother. It later turned out she was her own mirror self and when she died as a little girl, her brother had made a deal so her mirror self could live in her place and she forgot. It was hinted she had feelings for Shinji but nothing ever happened.
In 555 (2003), a member of the Ryusei School who was sent the Faiz Gear and Auto Vajin by Hanagata, but was unable to use them because she's not a Orphnoch. She is not as good-willed as she initially appears, and will often agree with Takumi when Keitarou suggests they use Faiz to help more people. She always believes the good in her friends, even Sawada and Takumi after she found out they were Orphnochs as well.
In Blade (2004) she is one of the few remaining BOARD members after it was attacked by the released Undead. She helped the Riders by detecting the Undead on her Undead-searching computer. She seems to care for Kenzaki very much.
In Hibiki (2005), the elder of the Tachibana sisters. A very intelligent individual, like her father. She is usually acts a field support for the Oni providing supplies and extra disc animals. She is liked by Ibuki, though early on she just ignores him. She previously played Succubus in Dekaranger.
In Kabuto (2006), she was involved in the meteor crash seven years ago along with Tendou. Due to the accident, her parents were killed. During the epilogue, she returns to her normal life in working at the Bistro, and now has Juka as a sister.
In Den-O (2007), she is a Singularity Point and the first DenLiner passenger Ryotaro met, convincing him to become a Kamen Rider while explaining the uses of the Rider Tickets and the Rider Pass. She keeps the Taros in line, especially Momatoras. She later regresses in age when the timeline changes. She is later to be revealed to be the daughter of Den-O's sister and a version of Zeronos.
In Kiva (2008), she is a famous model who is a Fangire Hunter and her duty was to hunt down the Fangire as well as finish her mother's unfinished affairs, such as the Rook. She meets Wataru and develops a liking towards him until she finds out he is Kiva. While Ixa had expressed his interest in her, she couldn't stand him. After the final battle was over, she would marry Nago a few days later. After marrying Ixa she tried to fight the future threat with Ixa.
In Decade (2009), she was Tsukasa's traveling companion, they travel in her grandfather's photo studio. She did seem to care for Tsukasa and cared for his well-being but never admitted she loved him. She kept him in control with a family pinch move to make him laugh, he used it on others too. She is the only lead girl to ever become a Kamen Rider, albeit only in a movie and not in the series.
In Double (2009-2010), Akiko Narumi is Shotaro's boss and landlord at the Narumi Detective Agency. She come to continue in her dad's place, then he found out he died in the Begins Night film. She serves as a personal assistant while on the job as well as keeping Shotaro on the right track. She has a black and white view of the world. Akiko harbors a crush on Accell, but her advances are ignored as he maintains a professional demeanor towards her.
In OOO (2010), she is very interested in in both professional wrestling and judo, and has superhuman strength. She began working at the Cous Coussier restaurant as the Greeed awaken, worried for her brother (his body has been hijacked by a Greeed). Eiji sends her a message stating Shingo is on an undercover mission and won't be home for a long time.
Let me know if I missed another important gal.