Kamen Rider: Temporary Belt Users

Here are random people who became Riders other than the main person or other ones took over or switch. A long long long list...

Albie (Saban's Masked Rider)
Albie the young kid brother to Dex, became the Masked Rider in one episode. I didn't see the episode, I didn't know it was just a dream sequence.

Goro (Kamen Rider Ryuki)
In Kamen Rider Ryuki, Goro was Kitaoka Shuichi's devoted loyal servant, he pretended to be Zolda to trick Shinji but when Kitaoka died, Goro transformed into Zolda, being killed off by Ohja.

Kōichi Sakakibara (Kamen Rider Ryuki)
In the 13 Riders Special, Koichi was Ryuki and gave the deck to Shinji before he dies.

Shinji (Kamen Rider Knight)
In the 13 Riders Special, Shinji gets the Knight deck when Ren dies.

Kyōsuke Tokumoto (Kamen Rider 555)

After Shindo's death, he tried to put on the Kaixa belt, but couldn't out of fear because of the other users' deaths. After the Delta belt was discovered and he used it, he believed the belt was rightfully his because of the belt's side effect. Not only that, he also gained some power from the belt.

Kiyotaka Nishida (Kamen Rider 555)
Temporarily used the Kaixa belt in episode 11 and 12. With the Kaixa Gear, he destroyed Crocodile Orphnoch's first life. But he soon died in Mr. Masuda's arms from the belt's side effect.

Takahisa Shindō (Kamen Rider 555)
Used the Kaixa belt in episode 12 to fight the Crocodile Orphnoch, despite others' protest, but the Orphnoch knock the belt off him, forcing him to revert and disinterage from the belt's side effect.

Rina Abe (Kamen Rider 555)
Rina Abe is Shuji Mihara's friend, former classmate and co-worker in the orphanage. She wore the Delta armor but was kicked out of it a few seconds later.

Saya Kimura (Kamen Rider 555)
A "nekojita" much like Takumi, Saya briefly wore the Delta Gear. Eventually she found her way to the Kikuchi Dry Cleaning shop and worked part-time there. She waited to see what kind of person Takumi was before deciding to hand over the Delta Gear to him.

Yuji Kiba (Kamen Rider 555)
Yuji Kiba is Horse Orphnoch and temporarily wears the Faiz Gear (when Takumi become the Wolf Orpnoch) and The Kaixa Gear (near the end of the series).

Kamen Rider The Bee (Kamen Rider Kabuto)
Sou Yaguruma, Shun Kageyama, Arata Kagami, and Masato Mishima all became Kamen Rider the Bee.

Yuri Aso (Kamen Rider Kiva)
Yuri Aso is the founding member of the Fangire Hunters. She became Kamen Rider Ixa in the Kamen Rider Kiva movie (2008).

Megumi Aso (Kamen Rider Kiva)
Megumi Aso is a famous model and the daughter of Yuri living in the year 2008. Megumi became Kamen Rider Ixa in the Kamen Rider Kiva movie (2008). In the S.I.C Hero Saga side story King of Vampire, Kivala chooses Megumi to become Kamen Rider Kivala.

Kengo Eritate (Kamen Rider Kiva)
Kengo Eritate is an independent rock guitarist and vocalist from Tokyo who lied about coming from Osaka out of shyness. When Nago is injured in action, Kengo eagerly takes his place as the Ixa System's user.

Maya (Kamen Rider Dragon Knight)
With Kase vented, Eubulon made her into Kamen Rider Siren. She later gave her Advent deck back to Kase when Kase returned. Eubulon gave her magical martial arts fighting knowledge, that we are to assume went away once she gave up the deck.

Akiko Narumi (Kamen Rider W)
Akiko Narumi is Shotaro's boss and landlord at the Narumi Detective Agency. Episode 30 she becomes W, well the Joker part. I didn't see the episode, I didn't know it was just a dream sequence.

I am sure there are so many so many more, just leave a comment and I will post it on this post.