Kosuke Nitoh

Kosuke Nitoh (仁藤 攻介) is the secondary rider of Kamen Rider Wizard. Kamen Rider Beast make his first appearance in the 2012 Movie War film, Kamen Rider × Kamen Rider Wizard & Fourze: Movie War Ultimatum. Referred to as "the ancient magician" (古の魔法使い, Inishie no Mahōtsukai) by the Wiseman due to his ancient Belt, he needs to absorb the mana of the defeated Phantoms in order to regain his strength.

Kosuke Nitoh
Kosuke Nitoh is a student archeologist with a flamboyant attitude who discovered the Beast Driver by accident during his expeditions in ancient ruins. He also has an unusual liking for mayonnaise, constantly carrying a bottle of it with him and putting it on any food he eats, including doughnuts. Haruto and Shunpei refer to him as "Mayonnaise" because of this unique habit. When he finds Phantoms or Ghouls, he refers them as his "food" due to his ability and need to absorb Mana from them. It's shown that when Haruto finds Phantoms and tries to defeat them, Kosuke will get angry for interfering with his "feeding time". He also constantly leaves everyone around him speechless due to his tendency to interrupt others and express his thoughts without breaks.

While he sees Haruto as a nuisance, he is nevertheless willing to cure him of the poison injected into him by Manticore through the use of Dolphin Mant's powers.

Later, Kosuke revealed to Haruto, Shunpei, and Rinko that he received Kamen Rider Beast's powers from Beast Chimaera. The Beast Chimaera make a contract with Kosuke, granting Kosuke his magic. In exchange, Kosuke must absorb mana by defeating Phantoms and ghouls, or he will be eaten by Beast Chimaera.

Rise of Beast
Kosuke meets Haruto, Shunpei, and Rinko by chance during a Ghoul attack, when he was about to have his lunch. Kosuke is then mistaken as a Gate by Haruto. He decides to keep eye on him, much to Kosuke's chagrin, who sees Haruto's intervention to have "ruined his meal". However, the Phantom Manticore was also sent by Wiseman to attack Kosuke. Haruto transforms during the attack into Wizard to guard Kosuke, while Shunpei and Rinko takes Kosuke away from the scene.

Meanwhile, Kosuke, annoyed by the trio's attempt to keep him safe, ran off on his own and met a tarot fortune teller, whom he had taken an interest in. Kosuke then decide to let the fortune teller predict his future. The fortune teller tells Kosuke that his future is bleak, but Kosuke, due to his super positive mindset, retorts in a cheerful manner that wherever there is a danger, there is an opportunity waiting. He further irks the fortune teller by saying that even hope can be found within despair. The fortune teller is later revealed to be Manticore when Haruto arrives to save Kosuke, and chokes Kosuke as Manticore revealed himself.

Instead of feeling afraid like victims usually do, Kosuke is overjoyed instead. Due to Haruto's inability to fend off Manticore in his current state, and that Kosuke desperately "needs to feed", he transforms into Kamen Rider Beast. After making short work of the Ghouls (and devouring their magical energy), Kamen Rider Beast takes on Manticore and uses Chameleo Mant to attack the Phantom unseen and gives him a hard time.

When Haruto tries to warn Kosuke of Manticore's poison attack, Kosuke was caught off-guard and the Phantom successfully poisons Kamen Rider Beast. However, Beast uses Dolphi Mant shortly afterwards to cure himself and Haruto, further angering Manticore. Kamen Rider Beast then uses Falco Mant and overpowers Manticore, and finally finishing the Phantom off with Falco Saber Strike and absorbing his Mana.

Though greeted by Haruto over his victory, Kosuke says nothing to reveal his origins as Kamen Rider Beast, but warns Haruto in a peculiar manner to never again interrupt his "meal".

Magical Food
Kosuke was seen enjoying a barbeque in a temple, but was asked to leave by the temple elderly. He then sees an artist, Hiroshi Oikawa, being attacked by the Hydra Phantom. Haruto also arrives at the scene and immediately transforms into Kamen Rider Wizard to engage the phantom. Since he was the first to spot the phantom, Kosuke is annoyed by Haruto's interference yet again, and he henshins into Kamen Rider Beast to proceed to fight Hydra Phantom as well. Upon realisation that he has to fight two magicians at once, the cowardly Hydra Phantom decided to escape in the midst of Wizard and Beast's bickering over who should fight the phantom. Kosuke then accused Haruto, saying that it was his fault that his "meal" escaped again.

Taking a break at the donut shop, Haruto is curious about how Kosuke obtained Beast's powers. Kosuke then tells Haruto and friends about the story regarding him and Kamen Rider Beast. Apparently some time ago, Kosuke was an archeologist who went on expedition discovering some ancient ruins. Soon, he chanced upon the Beast Driver and met the Beast Chimaera, who then made a pact with Kosuke. To his utter surprise, several Ghouls were released within the cavern's vicnity, but he managed to transform for the very first time and defeat them as Kamen Rider Beast.

Some time later, Haruto is alerted that the Hydra Phantom is going to attack Hiroshi once again. On the other hand, while sending Griffin to look for the phantom, Kosuke meet Misa, who is actually trying to make use of him by lying that Haruto is trying to destroy the Hydra Phantom so that he can absorb the phantom's mana. Acting on impulse and choosing to believe Medusa despite her being a phantom, Kosuke rushed to the scene of battle quickly to stop Wizard. While Wizard in his Water Dragon form is engaging the Hydra Phantom, Kosuke then transforms into Kamen Rider Beast with Buffalo Mant in an attempt to stop the battle. In the midst of the skirmish, Kosuke wants to know which side, Wizard or Phantom, can offer him a better deal in providing him the mana of phantoms.

Mantles
Unlike Wizard, Beast doesn't have access to alternate forms. Instead, he uses single shoulder mantle armaments called Mantles (マント, Manto) to help him gain an advantage over his foes.

Devices

 * Beast Driver - Transformation device
 * Dice Saber - Kamen Rider Beast's weapon
 * Beast Colors - Gives access to Beast's mantle changes
 * Griffin PlaMonster - Beast's familiar

Portrayal
Kosuke Nitoh is portrayed by Tasuku Nagase (永瀬 匡).

As Kamen Rider Beast, his suit actor is Eitoku (永 徳), whom among the majority of his various roles are Heisei Kamen Riders. He is also the suit actor for The White Wizard. He is the first Secondary Rider of the Neo-Heisei era, since becoming Kamen Rider Diend suit actor. Also in Movie War Ultimatum, his suit actor is Jun Watanabe (渡辺 淳). He also Phoenix Phantom's suit actor.

Trivia

 * He is the first secondary rider to be colored gold, and the fourth in general after Agito, Odin, and Caucasus.
 * He is similar to Kamen Rider OOO in his Latorartar Combo, as they both are yellow/gold in color and have a lion motif.
 * Him using a Griffin as a PlaMonster is probably alluding to how Griffins have the body of a lion.
 * Beast's ability to absorb mana from the Phantoms echoes Beet and Stag Buster, the additional heroes of Tokumei Sentai Go-Busters, the first Super Sentai show that Wizard aired alongside, whose respective mechas (BC-04 and SJ-05) are able to siphon Enetron from Enetron tanks and Vaglass MegaZords.
 * His unsual liking to mayonnaise is similar to Toshiro Hijikata from Gintama.
 * Kosuke is the second individual to be mistaken as a Gate after being attacked by Phantoms (obviously, he's actually a full wizard), the first was Satoshi Ishii from episodes 14 and 15 who was revealed to be the Phantom Lizardman, the real Satoshi having perished when Lizardman was released along with the other Phantoms in the Sabbath.
 * According to Kosuke's actor, he wanted to call the Phantoms "Yummies" because of how Kosuke eats their mana, but the director told him no, in order to avoid confusion with the monsters of Kamen Rider OOO.