|Philip is a mysterious young man who possesses the true Gaia Memory, referred to as the "Chosen Child" whose powers were used to create the Gaia Memories and whoever possesses him would rule the world. After being rescued by Shotaro and Sokichi Narumi from the Museum, receiving his new name from the latter, Philip aids Shotaro in order to atone for being used as a puppet by his captors. He investigates Dopant activity through his ability to access the Gaia Library and its Infinite Archive, a metaphysical realm and Akashic records in the form of white room filled with an endless number of books on an endless number of bookshelves. However, due to having no memory of his life before being rescued and the need to keep his location secret from the people Shotaro rescued him from, he usually spends his time in the Detective Agency's secret hangar, reading about common topics that he finds interesting. Sometimes his unintentional focus on a topic will turn into an obsession that overwhelms him until he knows everything about that subject, often getting in the way of ongoing investigations. He fights the Dopants with Shotaro, allowing his consciousness to separate from his body to fight as Kamen Rider Double, providing the Gaia Memories for the right side of Double.
He eventually discovers that his real name is Raito Sonozaki, the third child and only son of the Sonozaki family, after his mind was altered by Satoshi Yamashiro under his father's wishes for Raito/Philip to have no memory of his name and family (which explains why the pages on his family were torn out when he was attempting to look them up), as well as discovering that his mother is Shroud. It is also revealed to him that Raito actually died twelve years ago in a freak accident, and he was revived as an avatar for the true Gaia Memory to save him. After he is joined with the Gaia Memory once again, he becomes a part of Wakana until Shotaro is able to overcome his fear and pull Philip's mind (along with the Cyclone Memory) and body (with the Xtreme Memory) out of ClayDoll Xtreme. However, this event makes Philip's body unstable to the point that should he transform into Double, again, he would break up into data and completely be absorbed by the planet. Saving their final transformation to save Wakana and fight the Utopia Dopant, Philip has Shotaro promise to him that he will still fight for Futo even if he is gone. After Kazu is defeated, Philip bids his final farewell to Shotaro as he fades away once the transformation is ended. Philip left Shotaro a gift containing a Lost Driver belt and the blank book he used to access the Gaia Library containing a message thanking Shotaro. However, after Wakana learns that Philip sacrificed his own physical existence to save her, she initiates her own Gaia Impact to bring back the Xtreme Memory which slowly reconstructs Philip's physical body. Before returning, Philip has one last moment with his family who tell him that they will be watching over him. Once revealing himself a year after his physical form is completely restored, Philip resumes protecting Futo with Shotaro as Double. He, as well as his cat Mick, are the sole surviving members of the Sonozaki Family.
FangJoker is a form that Kamen Rider Double first uses to escape from Philip's captors during the events of the Begins Night as seen in Kamen Rider × Kamen Rider W & Decade: Movie War 2010. As the Fang Memory was designed for Philip, Shotaro's consciousness transferred into Philip's body for the transformation. Because of this, when Shotaro speaks, Double's left eye flashes. This allows Philip to go out into the open and utilize Double in case Shotaro is not around to help. However, the transformation is extremely taxing on Philip's body, and he lost control of himself in this form during its first two uses. In addition, they cannot stay in the FangJoker form for too long due to Philip's weaker body. By activating the Fang Memory's horn between one and three times, FangJoker is able to summon a bladed weapon that forms on various portions of FangJoker's body. One time summons the Arm Saber on FangJoker's wrist to be used as a close combat weapon; two times summons the Shoulder Saber, which can be removed from his right shoulder to be used as a dagger or boomerang-like weapon; three times summons the Maximum Saber on his right ankle. This last Saber is used to perform FangJoker's Maximum Drive: the flying roundhouse Rider Kick Fang Streiser, in which a projection of the Fang Memory's head is shown "biting" the opponent when Double comes in contact.