Kotaro Sakuma

"Big Star! Koguma Skyblue"

- Koguma Skyblue's role call

Kotaro Sakuma (佐久間 小太郎) is a young boy who defied Jark Matter's occupation of Earth with his younger brother, Jiro Sakuma. Inspired by Stinger, Kotaro decide to join the Rebellion. He later gains his own personal Kyutama from Shou Ronbou's predecessor, Big Bear to become Koguma Skyblue (コグマスカイブルー), the eleventh member of the Kyurangers. His alternate form is called Ooguma Skyblue (オオグマスカイブルー), though what this form is is currently unknown as it has yet to be seen.

Aged 13, Kotaro was born on May 5th.

Meeting the Kyurangers
While the adults refused to defy Jark Matter to make it easier for everyone, Kotaro and his brother refused to accept the oppression, leading to them being caught up in the fight between the Kyurangers and Jark Matter which resulted in the boys being caught by Stinger who, though ordered by Eriedrone to kill them, decided instead to keep them hostage so as to force the eight Kyurangers to surrender their Kyutamas. The brothers' bond reminded Stinger, secretly a Rebellion spy operating within Jark Matter, of that he once had with his own older brother and so he told them to watch out for each other. Ultimately, while Eriedrone intended to kill the brothers regardless, Stinger dropped his facade at the last minute, saving the boys before joining with the Kyurangers to take down Eriedrone, raising the hopes of the previously disillusioned adults.

Pursuing the Kyurangers
Some time after the Kyurangers victory, Kotaro follows the Kyurangers actions and locations, even going as far as getting into areas which were under Jark Matter's control. As Garu, Raptor and Stinger headed the last of the five Moraimarz they've taken down, he got caught by the Indaver but was saved by Stinger. He then reveals his intentions to join the Kyurangers to Stinger. Stinger brought Kotaro to the Orion, much to Shou Ronbou's surprise of bringing a kid in. As the Kyurangers were in a dire situation, Shou decided to get into action, telling Kotaro to stay and keep watch of the Orion. He even don't know what was going on when Lucky asks for Pega-san to be sent to them.

After the Kyurangers made a hasty retreat, save for Spada, Kotaro still begs Shou to let him join the Kyurangers. He was left behind again when Shou went out alone in the rescue mission and the other Kyurangers finding the commander. After a successful mission and Shou suggesting a new name for the team due to having a 10th member, though rejected by the other Kyurangers, Kotaro still insist that he should be the 11th Kyuranger. This went to the point the Kyurangers were annoyed by his persistent attitude.

Joining the Kyurangers
Kotaro stays behind in the Orion and tries various Kyutamas to see which one was compatible with him, though Champ mentions that in order to become a Kyuranger, the Kyutama has to choose the user. At that time, the Ooguma Kyutama glows. When Lucky brought him back to Earth, Kotaro reveals that he took the Transformation Controller Seiza Blaster with him without the Rebellion knowing, much to Lucky's shock (similar to how Lucky took the blaster and going off with it in the very first episode). Lucky chases Kotaro to get the blaster back but were attacked by Daikaan Mozuma. The Ooguma Kyutama in Kotaro's bag flew to Lucky's blaster, resulting in Big Bear's spirit attacking Mozuma and possessing Kotaro. During their search for Mozuma, Kotaro reveals to Lucky and Big Bear that he decided to become a Kyuranger to be the shining star of hope that will save Earth. He also made this promise to his younger brother Jiro. They found Mozuma and Kotaro attacking them even without an ability to transform. Seeing Kotaro's resolve, Big Bear entrusts the Ooguma Kyutama to Kotaro and the Koguma Kyutama was created. He transforms to Koguma Skyblue, using the scarf to fight and prevents Mozuma from getting away before finishing with an All-Star Crash just in time for the Orion to evade the Sun. After returning to the Orion, Kotaro dons a cyan team jacket as the 11th member of the Rebellion.

When learning about the legendary ship Argo and all 3 kyutamas needed for it were on Earth, Kotaro was elated to be part of the operation, though he got disappointed that his kyutama did not come out of the Kyulette. He manages to sneak into the Shishi Voyager, much to the group's surprise. Upon encountering Jark Matter troops, the group fights them off, with Kotaro's childish fighting skills got into Stinger's nerves. When the team were trapped underground, Kotaro learns from Stinger about his reasons to fight and Scorpio. In the battle against the enlarged Deathworm, Kotaro was unsure if he was able to summon a Voyager, though with the encouragement of Stinger, he summons and operates the Kuma Voyager. He uses the Koguma Voyager to save Lucky in time.

Departing the Kyurangers
During the operation to free one of the districts from Jark Matter's control, which involved citizens turning into zombies due to Scorpio's venom, Kotaro joins the main team to cure the citizens. He even questions Stinger into thought of dealing with Scorpio and reasons of mentioning him into detail. After dealing the situation, the Kyurangers holds a party for Kotaro before his departure to the Rebellion main headquarters to train.

Kotaro contacted Shou Ronpo as Rebellion HQ transported the Hikari Kyutama to the Orion to help the Kyurangers overcome Shaidos. Reporting that he was making good progress, Kotaro informed the team that Champ was still awaiting a series of assessments as his body was being repaired while Stinger continued to feel guilty for his injuries; Kotaro was entrusted by Shou to watch over them.

Seeking the Kyutama of the Tokei System, the Kyurangers came to the planet Toki and turned the 12 keys needed to unlock it within 30 minutes. Approaching key XI, Kotaro was met by his late mother, Akemi Sakuma. He was conflicted when he found that turning the key would make her disappear, and she was concerned for her son's well-being as a warrior fighting Jark Matter. However, upon seeing his teammates' plight against Tecchu, Kotaro ultimately turned the key, telling his mother that he would save the universe.

Personality
Kotaro has a high sense of bravery, not being afraid of defying Jark Matter. He is also protective of his younger brother and the people dear to him. After seeing the Kyurangers being able to bring hope to others, especially sharing a common brotherhood bond Stinger had whom Kotaro inspired, Kotaro went all the way to follow them, even intending to join the Kyurangers. His wish became true after gaining the trust of Big Bear and creation of the Koguma Kyutama to become Koguma Skyblue.

He calls Hammy 'Old Lady', much to her annoyance. Despite being the youngest member of the team, he has a mature personality, able to make correct decisions and helping the older members to reconsider their thoughts and reasons. He also has a strong determination to save Earth as the shining star of hope. However, due to his inexperience in battle, childish methods and tendency to butt into things without a strategy, he was sometimes a hindrance to the team. Despite that, Kotaro improves himself in fighting against the evil space empire.

Koguma Skyblue
"Koguma Kyutama! Seiza Change!"

- Transformation announcement via Seiza Blaster

Arsenal
 * Kyutama
 * Koguma Kyutama
 * Kyu Buckle
 * Transformation Controller Seiza Blaster
 * Kyu Spear -,

Mecha
 * Kuma Voyager
 * Koguma Voyager
 * Ooguma Voyager

Attacks
 * Polaris Crush (ポラリスクラッシュ): Koguma Skyblue performs a powerful blast attack with the Seiza Blaster.
 * All-Star Crash (オールスタークラッシュ): Koguma Skyblue performs a powerful blast attack with the Seiza Blaster alongside his fellow Kyurangers.
 * Kyuren All-Star Crash (キュウレンオールスタークラッシュ): An upgraded version of this finisher with the first 11 Kyurangers.
 * Ultimate All-Star Crash (アルティメットオールスタークラッシュ): An upgraded version of this finisher with the all 12 Kyurangers.
 * Polaris Impact (ポラリスインパクト): Koguma Skyblue performs a powerful slash attack with his own Kyu Spear.
 * All-Star Impact (オールスタースインパクト): Koguma Skyblue performs a supercharged energy attack with the Kyu Spear alongside his fellow Kyurangers.

Appearances: Kyuranger Episodes 10-13, 20-22, The Ghess Indavers Counterattack, Kyuranger 23, Transformation Lessons (Special), Kyuranger 25, 26, 28, 34, High School Wars Episode 4, Episode of Stinger, Kyuranger 37, 39

Portrayal
Kotaro Sakuma is portrayed by Shota Taguchi (田口 翔大), who previously portrayed the teenage Chihiro in the second season of ''Kamen Rider Amazons. His suit actor as Koguma Skyblue is Naoko Kamio (神尾直子, Kamio Naoko'').