Despite his role as a villain, Kaku is surprisingly honest even to his enemies and even more so than Fukuro though he doesn't seem to take insults well. As soon as his hybrid (giraffe/human) form was mocked, he sliced the Tower of Justice in half in a fit of embarrassed rage.
He also talks like an old man, as remarked by several characters, despite being only 23 years old.
After being revealed as one of the Water 7 CP9 agents, Kaku's personality becomes rather cold and dispassionate in contrast to his character on Water 7, where he was mainly seen laughing and smiling. Despite this, he often taunts his opponents during battle, ate a Devil Fruit for the fun of it, and exclaimed "This is fun!" during his life-and-death battle with Zoro. He also seems to place a chance to fight over easy accomplishment of a mission; when Zoro and Sogeking were cuffed together, he wished to help them, so he wouldn't be forced to share his target with Jabra. When neither agent had the key, both agreed that the prey was up for grabs. As Kaku speaks like he is older than he truly is, he uses the term "washi" (meaning "I", though used mainly by elders), rather than "ore" (also meaning "I", a masculine term used by young adults).
Kaku showed great sportsmanship after his defeat where he laughed at Zoro's comment of working in a zoo. It's led to believe that he is not as cold-hearted or evil as he appears, as he remarked "too bad" when Zoro passed Paulie's message of "you're fired", handed over his key without Zoro having to say a word, and showed apparent regret towards the carefree life in Water 7 he had left behind.
CP9's Secret Mission
Five years before the current events, four members of the secret organization CP9 took guises in the city of Water 7. Kaku as part of his mission infiltrated Galley-La Company by applying for a job as a shipwright, in the attempt to retrieve the blueprints for Pluton from Iceburg, mayor and former pupil of legendary shipwright, Tom. Kaku became infamous in the Galley-La Company, known notably for jumping off tall areas, clearing massive distances, earning the nickname: Yamakaze (山風, "Mountain Wind") and a respected member of the group. He gained the identity of a ship mechanic and diagnostic. Two other members Rob Lucci and Kalifa became a treenail specialist and personal secretary respectively, while the last member Blueno became a bartender. While gaining the city's trust and respect, they tried to get close enough to take the Pluton blueprints in Iceburg's possession. Unknown to them, a year later, Iceburg had passed the blueprints to Tom's other pupil: Cutty Flam, better known as Franky.
Kaku bears bad news to Luffy and Nami.When the Straw Hat Pirates arrived in Water 7, they needed repairs to their ship. However, due to a tipping off from Admiral Aokiji, a convenience in the form of "Demon Child" Nico Robin presented itself, and they blackmailed her to give a fake assassination attempt on Iceburg's life to frame the Straw Hats, and leave them to be taken to Enies Lobby, under the threat of a Buster Call attack. Meanwhile, the Straw Hats themselves pleaded for Galley-La Company to fix the Going Merry. When Iceburg asked Kaku to perform a diagnostic on the ship, Kaku confirmed that the ship is irreparable due to a broken keel and will sink in their travel to the next island.
However, at nighttime, an "attempt" on Iceburg's life, frames the Straw-Hats, with Galley-La company attacking them in the middle of a battle between Luffy and Franky, the next morning. During the fight Franky called Kaku Yamazaru ("Mountain Monkey") to mock him. At the evening, this time, the Water 7 CP9 group planned to steal the Pluton blueprints and invaded Iceburg's mansion. The four wore disguises, Kaku and Lucci pretending they were defeated by the assailants by using one of the defeated foremen to wear their gear. However, they realized that the blueprints were not with Iceburg, but learned there were with Franky by reading his pulse, when Lucci deduced he was Cutty Flam and one of Tom's pupils. After this, they left and to leave no evidence to their attack and their identities, they set Iceburg's mansion on fire, hoping to kill him in it. However, in the last minute, Iceburg and the Galley-La foremen were saved by Chopper. However, this was no major setback as CP9 had Nico Robin and know Franky's possible whereabouts.
After discovering Franky's secret hideout, Kaku sent the Going Merry into the midst of the Aqua Laguna and to its apparent demise, further reiterating that although his Water 7 guise was false, his analysis was genuine and the ship cannot sail. After capturing Franky and Usopp, they set out for Enies Lobby on the Puffing Tom.
CP9 vs. Straw HatsKaku facing off against Zoro in the Tower of Justice. Returning to Enies Lobby after five years, he and Kalifa were given Devil Fruits by Spandam and under the urging of Lucci, ate them. Although he was initially going to face off with Zoro, he ended up causing the floor to cave in by transforming into his full giraffe form instead of his half-giraffe form. The two fell into the room where Sogeking and Jyabura were fighting, and were involved in that fight as well. Jyabura started to break out in uncontrollable laughter after seeing Kaku's full giraffe form. Kaku manages to transform into his half-giraffe form, but this only makes Jyabura laugh more hysterically and makes Sogeking accidental handcuff Zoro and him together. All this mocking eventually gets to Kaku and, in anger, he uses an attack that slices the tower in half where it remains lopsided for the reminder of the arc. After a bit of arguing with Jyabura on whose prey is whose, the two agreed to work together to defeat the pirates and escape the oncoming Buster Call. Zoro however managed to hold them at bay by using Usopp as a sword (moreover having Usopp hold one of his swords while he uses Usopp like a hilt) until Nami frees them of their handcuffs. As Zoro and Kaku continued their one on one match, it seemed Kaku had the upper hand thanks to improvising with his devil fruit and his swordplay until he was defeated by Zoro activating a new power called Asura. Beaten but honorable, he gives Zoro his key before he passes out. It should be noted that Kaku held the important key to unlocking Robin's handcuffs.
CP9's Independent Report
He appears on the third cover in the report, sitting, and apparently severely injured, having been carried by Jyabura along the Sea Train tracks. He had arrived at St. Poplar and is shown pretending to be a "giraffe slide" while in his full giraffe form for children to ride, in an effort to help CP9 raise money for Rob Lucci's medical bills. Afterwards, he was at a café with Fukurou, Blueno, and Jyabura waiting for Kalifa and Kumadori to return with the shopping, he along with Fukurou is laughing at Jyabura. Later, he is seen as the scorekeeper while the others are bowling. However, their bowling match is interfered when the Candy Pirates ransack the island, so he and the rest of CP9 quickly dispatch them on their ship. Kaku was shown fly-kicking one of the pirates as the crowds cheered. However, since Lucci crushed the skull of the captain, the crowd was horrified and CP9 realized they can't stay in St. Poplar, so they shipped off to their homeland, where they watch a new generation of CP9 being trained. Captain Very Good arrives to capture them, but they refuse to let him disturb their old home.
As CP9 battled, and eventually defeated the Marine troops led by Very Good, Lucci contacted a physically crippled Spandam using a Den Den Mushi, and promised to come for him very soon. With that CP9 began their aim to eliminate their former director by leaving their homeland and setting sail.