Kamen Rider Fifteen appeared in 2014 TV special called Ressha Sentai ToQger Vs. Kamen Rider Gaim Spring Vacation Combining Special. His real name is Ren Aoi (葵 連 Aoi Ren) who is a mysterious man that transforms into Kamen Rider Fifteen (仮面ライダーフィフティーン Kamen Raidā Fifutīn), created by the Badan Empire in order to defeat the Kamen Riders. He is the father of Shuu Aoi and husband of Saki Aoi.
Ren Aoi is a family man when his son was alive. After his son died in an accident while having to do with meeting his wife at a lighthouse, Ren falls into despair after losing his son. When his son is in fact alive, Ren joins the Underground Empire Badan, becoming their Kamen Rider to find his still alive son, but somehow got brainwashed into their pawn.
Kamen Rider Fifteen first appears to fight the ToQgers, but is interfered by Showa Riders 1, 2, and V3. The three riders manage to drive off Fifteen, as he enters through a darker version of a Helheim Crack. During the post-credits scene, he foreshadows the events of Heisei Rider vs. Showa Rider: Kamen Rider Taisen feat. Super Sentai.
After finding his son at Zawame City, Ren appeared in his normal identity, battling Gaim, Baron, Ryugen and Zangetsu Shin, transforming into Fifteen, able to defeat Zangetsu Shin then later Ryugen, thanks to Heisei Rider Lockseed he use as Wizard Arms. His battle with Gaim is interrupted by three very first Showa Riders, leading Gaim to escape with Shu and Mai to safety. He found his son again in his home destroying the message that Shu made for his mother and tells him to come in order to use Shu's power for the Mega Reversal Machine. However, his son refused because Ren began to act strangely due to him being brainwashed, thus leading to Ren forcing his hand to take him away from Heisei Riders' protection. Upon his son finally caught nearby the lighthouse where his wife, Saki is, Fifteen managed to take his son away from the Heisei Riders and meet his wife. He is defeated by Kamen Rider Gaim 1 Arms after he was overpowered by his 1 Squash, destroying his Sengoku Driver and its Yomimaru, thus freeing him from Badan's grasp. After realizing for what he did when he was brainwashed by Badan, his son began fading away, but managed to take his son to the lighthouse to reunite with his wife, thanks to the Riders (mostly Showa Riders' Rider Syndrome teleportation) and seeing their son disappear to the top of sky as a light above them.