Jacqueline Muscle (ジャクリーン・マッスル) is the daughter of Harabote and sister of Ikemen. Jacqueline acts as a love interest for Mantaro, as well as the face for the Wrestling Federation, and she provides much of the publicity and attention to various matches throughout the series.
Harabote Muscle's daughter and Ikemen Muscle's younger sister. She is beautiful and a natural showman, but prefers violent matches and can be antagonistic to other characters (she can even make Ikemen cry). She becomes kinder after the Kinniku Mantaro vs. Kevin Mask match, learning what it means to be a Seigi Chojin and the power of friendship.
In the manga, she is very bloodthirsty. In the US dub, this blood-thirst became a desire to see interesting matches and cute men, and she becomes very flirtatious and wishes to date the winner of the Chojin Crown. Jacqueline initially favours Kevin Mask during his match with Mantaro, but changes sides when she witnesses his brutal nature. This shocks Ikemen when she stops to cheer and pray for Mantaro's safety, blaming herself for how he suffers. Jacqueline grows relieved to see him succeed.
In later manga arcs, Jacqueline initially becomes a background character, before taking on a strong supporting role and oftentimes becoming a main character in her own right. She is great friends with Rinko Nikaido and a huge fan of Mantaro Kinniku, and often provides valuable insight and support.
Ikemen describes her as "pretty on the outside", but "ugly on the inside".  Harabote Muscle describes her as "more temperamental than her brother" and not being very "ladylike", with images of her wrestling Ikemen as a young child and winning against him.  She displays signs of a sadistic personality, such as revelling in the sight of blood.  At times, Jacqueline can break her word, such as promising to sit quietly and stealing the limelight from Ikemen with her energetic personality.  It is revealed she often stole Ikemen's most cherished items, although this may be a biased statement borne from sibling rivalry and their age difference.  Jacqueline is also a natural showman, enough that her brother is envious of her skills, and is able to rile the crowds and attract attention. 
Jacqueline is seen to be a stickler for rules; she determines Ricardo's attack legal according to a loophole, when he attacks after the bell,  and she later disallows Mantaro to freely pick his second, citing the rules. In every instance, she knows the rules down to the paragraph and article.  She is seen to enjoy the spotlight, even using gymnastics at the start of the Kevin vs. Mantaro match to steal the bell from Ikemen, much to his consternation.
Chojin Olympics: The Resurrection Arc (Manga)
In the manga, Jacqueline first appears where she takes a VIP seat to view the battle between Ilioukhine and Destruction.  It is revealed she is Ikemen's sister, but that they have very little in common both in appearance and personality, and this causes some dismay to Ikemen.  It is her first time watching a wrestling match. Jacqueline becomes an unofficial presenter for the following match between Kevin Mask and Legox, in which she flirts with him and expresses a love for 'bad boys'. She later chastises Kevin after his victory, as the match is not bloodthirsty enough to her taste, and he responds by dropping the remains of his opponent into her hands, as a ‘gift for the princess’ in an act of mockery and contempt. 
It is not long after this that she amasses a small fan base, along with a love interest from Mantaro. Jacqueline intervenes for Ikemen when he is indecisive in reacting to a potentially illegal move, in which she decides that Ricardo did not break a rule by attacking Jade after he throws in the towel for his match.  Jacqueline is revealed to be the mastermind behind the Punishment X Ring, at which point both her father and brother express fear of her attitude.  Again, Jacqueline calls the match on Ikemen’s behalf and declares Ricardo the loser. In following chapters, Jacqueline flirts with Mantaro to inspire him into living up to the potential of his race and defeating his opponents. 
At the end of the Ilioukhine match, she steps literally upon the loser and shows exuberance at his defeat, and follows by trying to give attention to Kevin Mask who rejects her advances.
Despite her lust for blood and desire for violence, Jacqueline intervenes when Kevin Mask potentially breaks the rules and shows that she is ready to disqualify him, putting aside personal desire in order to maintain rule and order of the Chojin Olympics . Jacqueline expresses an interest in rules over money, enough to argue with Ikemen and willing to throw away all profits. She is shown distrustful of Croe's explanation of events, but ultimately concedes and allows for the match to continue. After this, Croe chastises her lust for blood, at which point Kevin collects his blood in his hands and attempts to throw it in her face - Mantaro blocks the blow. 
This leads to a key character change in Jacqueline: she learns obligation. It is now she realises the error of her ways and that bloody matches are not as fun as they seem, which leads to Ikemen and Harabote explaining to her that they must watch every match through to the end, and she grows determined to act as a better potential chairman. Jacqueline even goes on to support Mantaro. When Mantaro faces defeat, Jacqueline runs to his side and aids and nurses him. It is when she realises the real thrill of the match, as well as the emotion and stakes, that Harabote calls her his true successor.
Demon Seed Arc
Jacqueline gathers together a group of Chojin, featuring Kevin and Scarface, that she calls “The Army of Idols” in the English translation. She stops Mantaro from risking his life in a battle that cannot be won, while taking Buffaloman into her helicopter for medical treatment.  The Justice Federation’s “Airship Vision” arrives to allow them to see events elsewhere. Together, they watch the Hanzo vs. The Puppet match, which Jacqueline expresses admiration of Hanzo’s change of heart and concern at the Demon Seeds’ power.  She admits that since the Chojin Olympics final, she cannot see the sight of blood without cringing and recognises how precious blood is as a life-force.  Later she cheers on Hanzo during his match.  A couple of times – during this volume – Jacqueline can be seen booby-bashing Mantaro, using her chest to literally knock some sense into him. 
Later in the arc, Jacqueline observes that the more Barrierfreeman squeezes The Tattooman, that the more blood gushes from his wound, and she shows concern for his critical condition as a chojin.  Jacqueline provides emotional support to Mantaro, as she seeks to convince him to rest, but - when he flees to help Meat - Jacqueline uses her helicopter to follow him with the others in tow. She later arrives in time to save him from drowning, by providing him a flotation device. During the Demolition vs. Blood Evolution match, Jacqueline and Ikemen sense great danger due to their Chojin status and show visible fear. Jacqueline also provides emotional support to Rinko. 
Jacqueline takes Mantaro towards Kevin, after Kevin's match, into the ring so that they can shake hands, as she attempts reconciliation between Kevin and Mantaro. The two fight and part ways, causing Jacqueline to express dismay that they could not put their differences aside. Later, Jacqueline comes to their defence in Shibuya Square, when the public appear to turn against them, and she reiterates that nothing is stronger than the friendship between Chojin.  The rest of the arc sees Jacqueline as a supporting character, in which she provides minor exposition in regards to characters and matches, and offers support for the Seigi Chojin.
Ultimate Chojin Tag Tournament
Jacqueline appears with Ikemen on stage, in formalwear, as Harabote presents awards to the Seigi Chojin for their achievements over the past few arcs. She announces Kevin’s award personally, after Mars receives his award from Harabote.  She next appears listening to a speech by Meat about the nature of time-travel, sat front-row in the small crowd of Chojin . She appears with Rinko, Harabote, Meat, Ikemen, Tamaki, and Keiko, watching the attempts to create the ship on a small television in a meeting room. They discuss recent events, providing some exposition. They celebrate together on completion of the ship. Jacqueline volunteers and insists on travelling with the selected group of Chohin, so that she can take care of Kevin during their trip. Jacqueline is given a special seat behind team-leader Terry. 
Jacqueline insists Mantaro take her seat, after learning how dangerous the equipment could be if mishandled, hoping her seat – away from the equipment – will minimise potential risk. She then searches for her brother, but finds him in a secret room that he had built for their personal comfort. Suspicious of his now apparent excitement in travel and change of heart, she finds his gambling magazines and finds he plans on making ‘sure’ bets in the past to make profit, and proceeds to criticise him on his selfishness. They find Rinko as a stowaway in the luggage bay, at which point Jacqueline helps her onboard the ship and acts as her friend, even sharing her seat with Rinko. 
Jacqueline is left to pilot the ship with Rinko and Ikemen, after the Time Warp Eight abandon ship to save Robin Mask. Jacqueline leaves with Rinko into the two escape pods, leaving Ikemen to walk down the mountain with their luggage instead. Jacqueline and Rinko land amidst the various Chojin, where Harbote flirts and sexually harasses Jacqueline, unknowing of her identity, much to hers – and the audience’s – disgust. She then discovers Alisa has been harmed in place of Robin, hence why Kevin’s life is still in danger.  Jacqueline intervenes between the Time Warp Eight and First Generation to stop an ensuing fight, and soon realises that her brother has rang for help to save Alisa, after which Jacqueline pleads alongside Rinko with the Time Warp Eight not to fight the previous generation, before a new tournament is announced. 
Jacqueline often goes where Rinko goes, and together they usually follow Mantaro. On one excursion, they learn Terry has teamed with Robin, and Jacqueline provides some emotional support to Mantaro. They return with him to the ship, where they express shock as Seiuchin sides with Neptuneman, after which they go to a stage show in search of a strong Chojin, later revealed to be Chaos.  Jacqueline sides with Mantaro when they travel to Chaos’ home (Gachinko House), and she also lies alongside him to placate Chaos into allowing them entry and a free meal. They both chastise Rinko for being so critical. Jacqueline and Rinko arrive at the stadium for the prelimary matches, but grow lost in a search for their seats, at which point they are sexually harassed by Harabote (who takes a personal interest in Jacqueline). They decide to play along in order to obtain better views and seats, able to sit beside Harabote and Mayumi while the preliminary matches take place.  Kevin is soon presented at the side of the stage in his life-support machine, which is when Jacqueline realises that Ikemen neglected his duty to protect Kevin. She sees him in the crowd, at which point he pretends to be a beer seller, and expresses her frustrations.  Jacqueline later expresses pride and happiness at Harabote’s decision to split the preliminary battle royale onto two rings. 
After the match, Mantaro retires to recuperate. Jacqueline is there for support, along with Rinko, after Chaos runs away. It is at this point where Jacqueline and Rinko fight, until Mantaro pulls Jacqueline away and Meat pulls back Rinko. Jacqueline and Rinko then hide in Mantaro's dressing room away from the other Chojin that wait in the corridors, but Ikemen appears and Jacqueline attacks him for his selfish desire to gamble (which led to Kevin being kidnapped). Ikemen convinces her it was for selfless reasons, to which she shows admiration and respect for him. 
In volume 21, she dresses as a nun in order to sneak orphans into an upcoming match. The orphans are from Chaos' orphanage that wish to support him in his upcoming battles, which would otherwise be excluded, and she does this alongside the Mother and Rinko. Jacqueline is seen offering invaluable emotional support to Rinko in earlier chapters, alongside Ikemen, and helps to protect the timeline, too.
Ultimate Muscle: Anime Changes
In the American-commissioned anime, she is introduced asking Mantaro directions from a helicopter, after which he finds her casually waiting in Ikemen's dressing room. He teases the two siblings on looking so different, triggering a flashback from Ikemen about their shared childhood. Jacqueline is also made an explicit love interest, as opposed to a slow-forming friendship, and promises Mantaro a date should he win his match. 
Instead of being a bloodthirsty showman, Jacqueline appears now only as a wrestling fan that seeks the "cutest" wrestlers. Due to the matches now being simultaneous, she will watch the matches where her crushes appear, and she engages in an explicit love-triangle with Rinko, who is now jealous of Jacqueline (as opposed to their close canon friendship). Instead of being respectful of Ikemen, she also refers to him as a "sissy" and regularly argues with him.  Jacqueline also expresses a desire to be a wrestler, absent in the manga. 
Jacqueline can be seen purposely aiming for Ikemen's parachute, causing him great physical injury that lasts a substantial period of time for a Chojin, and refers to him as an "idiot", as she shows great contempt for him (as opposed to great respect and legitimate criticism in the manga). The two do appear to have a more classic sibling rivalry, but a highly antagonistic and aggressive one that is a far-cry from their relationship in the manga. 
- Her name can possibly be a reference to Jacqueline Moore/Miss Jacqueline from the WWE.
- Jacqueline often hosts the opinion poll special chapters  
- Developed a mild phobia of blood, cringes on its sight 
- Ikemen is one of the few people - in manga - to call her "Jacqui"