The Origin of the Ogre shaped back muscle:
The demon appeared when Yujiro stopped fighting for survival and instead for pleasure and amusement, the Ogre's symbolism is bloodlust usually Yujiro reveals this when going in for the kill often without even needing it.
Yujiro is a very cruel and arrogant man. He shows no mercy towards anybody, as he views mercy as a "weak" trait. He loves to fight and cause destruction to anything and everything he can. His whole life and mindset is dedicated to nothing but fighting and growing stronger with each person he defeats, sometimes killing them. He tends to kill people if pushed or enraged enough, and does not show care or remorse when doing so. He will also kill for pleasure or satisfaction when he feels like it. He has a deep resentment for those who are weak, and does not even think it's worth killing them.Yujiro is also very brutal, as shown when he ripped Ryu Kaioh's face off, and then slammed his skinless face onto the stone floor. Another example would be when he killed his own wife Emi by rupturing her eardrums, and then breaking her spine. He is a very arrogant man, believing, (and knowing as well) that he is the strongest living creature on Earth, and he feels inclined to do whatever he wants, with no one being able to stop him. He views everyone else as being insects compared to him, and seeing them as "the weak". He has a mental framework that thinks that everything should be obtained with pure strength alone. He does not believe in using weapons, as his body is the ultimate weapon which proves to be more effective and deadly than any kind of other weapon or technology.
Yujiro is the strongest character in the series, with his strength being said to equal that of an entire army or more.Yujiro was able to defeat the American military forces by himself during the time of the Vietnam War; Yujiro was 16 at the time. Some of Yujiro's greatest feats of strength include, pushing back 100 riot police, completely stopping the tectonic activity of an earthquake by punching the ground, tapping craters in concrete walls,easily knocking out a giant elephant that not even tanks could damage, cutting glass apart with his fingers, being tranquilized with a enough sedative to kill a blue whale and waking within an hours time, drinking a pint of 70% proof scotch in one gulp, and crushing diamond into dust. It is said that Yujiro's hits are equivalent to that of a nuclear impact, with even entire armies trying their best weapons to stop him, but to no effect. His body's muscles are so strong and durable to the point, that he is able to do things like breaking samurai swords and knives by gripping them, walking through bulletproof-glass walls, and shaking skyscrapers by merely making his muscles tense( it is implied that it is actually his body's aura). Another indication of his strength would also be his durability, (as durability and strength are in consistency with each other). Yujiro is durable enough to take a full impact hit from literally anything or anyone on Earth, without even flinching or being damaged whatsoever. He was shown to not be injured at all after falling from the top of a 50 story skyscraper. He has also never been shown once to actually have been hurt from any character who has hit him in the entire series (excluding Kaku Kaoih, who used the power of his own punches against him). Yujiro was shown to effortlessly overpower with just his fingers the likes of Oliva, who is strong enough to cause an interference in the GPS signal in cars all over the planet with a punch to the ground. His body is durable enough to tank things like bullets, fire, lightning strikes, blades, diseases of any kind, cannonballs, and even missiles, with none of these being able to even penetrate his skin or injure him at all. Yujiro was able to easily move at bullet speeds when he was just 16. Yujiro was shown to catch a titanium arrow that required 200 lbs of force to successfully project, and caught before it could leave the bowstrings from 9 ft away. He is shown to move at warp-like speeds, that are imperceptible to other characters vision, almost as if teleporting. He is able to easily outmaneuver any character in the series in terms of pure speed; with characters who are at mid - top tier level being able to traverse across hundreds of miles in a matter of hours on foot. He can strike with enough velocity that can break the sound barrier and rip through the air, causing extremely intense gusts of wind.
When Yujiro flexes his latissimus dorsi muscles, his back takes the shape of a demon's face, thus increasing his strength and combat abilities. Yujiro is constantly advancing his strength, becoming stronger with each person he defeats everyday, essentially "devouring" them, as he himself tends to say, all the while his "demon back" is exposed. A genius fighter he is known to have mastered all forms of unarmed combat. His repertoire apparently includes all of the well known fighting styles ( such as karate, boxing, taekwondo, jujutsu, wrestling, street-fighting, and so on) as well as some more esoteric ones (such as the udonde or uzunde style, an actual system used in the Motobu-ryu of karate). Since the age of sixteen, he has been fighting on the battlefields using just his bare hands, when he fought against both American and Vietcong in the Vietnam War. Since then, it appears he has obtained respect from the American forces, even going so far as them declaring amenity towards him with each new president. Yujiro also has a complete and full understanding of the human body and its anatomy, and essentially has complete medical knowledge on each organ and muscle and their respective function. It should be noted as well that, Yujiro has the extraordinary ability to instantly learn and master a fighting style perfectly, upon the first time seeing it.
Yujiro has no exploitable weakness
you could technically count Yujiro's pride but Yujiro has never lost a fight and his danger sense counters any mistake his pride would lead him to make.
Most of us know what Samurai Jack can do. he is a total badass and his feats I feel more than qualify him for this matchup. Not to mention his sword plays a key factor its unbreakable and cuts almost anything. however given Yujiro's own powers and skills i don't feel like Jacks blade gives him much more of an edge. so it should be a very fair matchup.