He was born into the Estraneo Famiglia along with Ken and Mukuro and was one of the children who were experimented on when the Famiglia invented the Possession Bullet and were as a result horribly persecuted by other Mafia Famiglias; however, this changed when Mukuro saved him and Ken by annihilating the entire Famiglia. Since then, they became Mukuro's loyal subordinates.
When Tsuna was running away with Futa from the Todd Famiglia members, Chikusa was seen at a vending machine as Tsuna ran by with Futa on his back, recognizing the latter as the Ranking Futa, and later on curiously gazing up when Reborn fired the Dying Will Bullet at Tsuna, as it made a rather loud sound. Afterwards, he returned to Kokuyo Land and reported to Mukuro that he found Ranking Futa.
List Countdown Edit
Chikusa first appears with Ken observing his battle with someone on the list. He later corners Gokudera in a Namimori Shopping District. He then engages in battle with the Vongola, but underestimates the third-ranked on the list, resulting in him taking on serious damage. He proves to have a very resilient body as he is able to take them and incapacitate Gokudera when he tries to save Tsuna. After he attempts to attack Gokudera's savior, Yamamoto, one of his Hedgehog Yo-Yos are sliced; when he loses it, he decides to retreat and take a shower.
Kokuyo Land Invasion Edit
For the first half of the Invasion, Chikusa is unseen, as he is recuperating from his battle earlier with Gokudera. However, he comes back in the latter half of the Arc to battle Gokudera on Mukuro's orders after the group (without Yamamoto) proceeds into the main building. He battles Gokudera until his side-effects kick-in; along the way, proving again, he can take large hits by being hit dead-on with dynamite explosions and only wobbling. At this point, Ken breaks through the glass, Gokudera is resting on, and deals a heavy blow to him. This allowed him and Chikusa ample time to finish him off. The two look on as Gokudera rests helpless at the base of a flight of steps. But, they allow him time to release Hibari who defeats them both off-screen.
Mukuro Face-off Edit
After Mukuro's Possession Bullet is revealed, Chikusa and Ken come back to aid Gokudera and Bianchi, as the four are being controlled by him. However, after Tsuna receives his X-Gloves from Leon, he easily defeats both of them. This ended Chikusa's involvement in the Arc, and his last appearance was when he was dragged off by the Vindice along with Ken and Mukuro.
Chikusa and Ken appear when Tsuna was told to buy drinks after he questioned who the Mist Guardian is. When he reached the shop, however, Tsuna immediately fainted when Ken surprised him from behind. After a short conversation with Reborn, he and Ken left as Ken says that Tsuna's Mist Guardian is too good for him. Chikusa later appear again in the gym where Mist Ring Battle took place, escorting Chrome. After Mukuro defeated Mammon, he changed back to Chrome who was exhausted and fell asleep. Chikusa and Ken left without her.
Chikusa and Ken watched Hibari's fight under Mukuro's order. During Gola Moska's rampage, they saved Chrome from the mines that were buried within the ground that she accidentally stepped on. This left them unguarded and trapped between Gola Moska and rifle pointed at them. Fortunately, Tsuna arrived in time, protecting the three of them from both attacks.
He and Ken secretly followed Chrome to the last match, but was found out by the Cervello and was told to stay and watch outside the battlefield along with Reborn, Colonnello, Basil, and Shamal until the match ends. He stopped Ken when he tried to save Chrome.
Battle with Glo Xinia Edit
Chikusa, along with Ken, makes a cameo appearance during Chrome's battle with Glo Xinia in a flashback. Ken was talking about their secret hiding hole and put gum in it, after which Chikusa commented that gum was not food. Chikusa, along with Mukuro and Ken, makes another appearance during Chrome's battle with Glo Xinia. All three of them appear as Real Illusions made from Chrome, with the help of her Vongola Mist Ring. He has recently appeared alongside Ken and M.M. after Fran freed Mukuro from Vindice. They then escorted Mukuro to Japan to help protect Uni from Byakuran.
He first appeared when he told Kyoko, Haru, and Bianchi that Chrome disappeared ever since Tsuna and the others comes back from the future. Later when Chrome returns, he notices that Chrome was teary eyed. He pointed this out to Ken, who tries to make her feel better by offering her snacks, to which she refuses, before breaking down and crying. He and Ken tries to figure out what's wrong with her. The next day, he and Ken met Tsuna and his friends outside of Kokuyo Land and informed them about Chrome's return and about her strange behaviour. When they returned, they were shocked to see that the whole Kokuyo building had disappeared. After the building appeared again, he and Ken were about to question Tsuna, but was interrupted by Chrome whom they realized has already returned to normal.
Later, he and Ken fought against Zakuro after they helped Mukuro escape from Vindice Prison. Chikusa and everyone else were watching the battle between Tsuna and Ghost, and then Tsuna's battle with Byakuran. After Byakuran was defeated, he was seen with Mukuro and Ken, calling out to Chrome, that it's almost time to go back to the past.
He was seen for the first time when he, Chrome, and Ken received a package which contained food from an unknown sender. He wonders who keeps sending the food if it's not Tsuna.
Search for Fran Edit
Chikusa, along with Mukuro, Ken, and M.M went to countryside of France called Jura to recruit Fran into their team. He strained and told Mukuro to calm down when the latter intended to attack Fran out of anger for calling him pineapple fairy. When Mukuro and Squalo managed to stop Fran, he began his insult by calling Chikusa glasses-fairy and his skin is like an eraser, much to Chikusa's annoyance.
1st Day of Representative Battle Edit
Chikusa is then later seen accompanying Mukuro as he went to Namimori to announce to Tsuna his "declaration of war."
2nd Day of Representative Battle Edit
During the second day of Representative Battle of the Rainbow, he, Mukuro, Ken, and Fran ambushed Team Reborn and Team Uni. He and Ken were disqualified from the battle after their wristwatches were destroyed by Colonnello's Maximum Rifle.
3rd Day of Representative Battle Edit
During the third day of the Representative Battle of the Rainbow, he and Ken rejoined the battle by using the two remaining spare wristwatches, allowing them to assist Mukuro and Team Reborn to fight the Vindice. However, when the Vindice's true form are finally revealed, he, Ken, Yamamoto, and Gokudera were easily knocked down by the Vindice, leaving only Mukuro and Tsuna to fight them. After the third battle was over, Chikusa and Ken was taken back to Kokuyo Land along with Mukuro to treat their injuries.
4th Day of Representative Battle Edit
He and the other Kokuyo Gang members along with the people Tsuna had talked to gathered at Tsuna's house to discuss plan to defeat Team Bermuda. After the battle was over, he, Ken, and M.M visits Mukuro, Chrome, and Fran at hospital. He is last seen beside Ken and Chrome when Hibari destroyed the wall of Mukuro's room.