WARNING, SPOILERS AHEAD
Real History Route
Initially, Rance was just touring JAPAN with Sill for an onsen trip with the ultimate goal of H-ing JAPAN's famed beauty, Princesss Aroma. Along the way,Rance was mostly just H-ing people. One day they stopped by a dango house, and met Nobunaga and Princess Aroma (who turn out to be way under the age limit for Rance) of the Oda Clan. Nobunaga and Aroma enjoy the lives of ordinary folks, which is why they often disguise as commoners and work in the dango (several balls with a stick going through them) place. Incidentally, dangos made by Aroma are... inedible... Nobunaga and Rance got along very well. Eventually Nobunaga lets Rance run the Oda house from behind the scenes, because Nobunaga doesn't want to deal with the politics at all, finding it too much a bother.
The Possession of Nobunaga
The Demon Xavier was previously sealed in 8 different gourds. Each gourd was entrusted to 8 different houses by Tenshiism to be guarded through the future generations. Nobunaga's monkey broke the one kept by Oda house, so a portion of Xavier's essence escaped, and possessed Nobunaga. Using Nobonaga's influence, Xavier began organizing a secret group of ninjas directly under his command and began seeking out the gourds kept by the other houses. Four of his acolytes (Seiryuu, Suzaku, Genbu, and Byakko) had their seals broken as the gourds were destroyed and rejoined their master. During this time, Xavier (in Nobunaga's body) publicly proclaimed to be ill, and moved to live in the Honnou Temple where his body may get more rest.
The Honnou Temple Incident
When Xavier got five out of the eight gourds, he brutally tortured and killed a Tenshiism monk. On her visit to the temple and Nobunaga, Princess Aroma witnessed this event. To prevent her from revealing this event to others, Xavier sent his minions to deal with her. After some time passed and Aroma did not return, Rance and the others went to search for the Princess. Eventually, Rance found her with Xavier's cronies outside the Temple and killed the cronies. Princess Aroma shortly after revealed the possession of her brother Nobunaga by Xavier. After hearing this, Rance organizes a force to fight the demon Xavier. This force caused Seiryuu to flee and killed Byakku. When they reached Xavier, they saw that Seigan and Kentarou were both just defeated by Xavier. Rance was able to seriously injure Xavier, Nobunaga's last remaining will partially regained control of his body and resisted Xavier, causing more damage to the body. At the end of the fight, Kentarou recovered, and attacked Xavier in such a way that allowed Xavier to escape through the window. Nobunaga's body was never found, but Princess Aroma (who was waiting outside) mentally heard Nobunaga calling out to her. This was the last part of Nobunaga that was left, which meant that Xavier was in full control of the body.
After the Honnou Temple Incident
After recuperating from the battle with Rance and the others, Xavier eventually appeared in South Africa, where the Shimazu brothers rule. Xavier found his daughter, Princess Black, who was a guest of the clan. When the demon was about to kill them, Princess Black begged for their safety. As a result of this, they pledged their obedience to Xavier, and their army was to be called the Demon Army. Through Princess Black's ability to break seals, two Forbidden Youkai were revived. One of the youkai was Stone Grandpa, who can grow larger when in pain. Xavier planned on sinking Japan with this ability. In order to supplement his army's strength, he used the ability of the other youkai, called Soul-Binder. Whenever the Soul-Binder infects a human, they become soul-bound and behave in a way similar to zombies. With this force, Xavier planned to conquer Japan. Fortunate (or unfortunately for those in the way), Rance's army traveled West to face the Demon Army. Eventually the group rescued Stone Grandpa. The final battle between Rance and Xavier took place at the Tenma Bridge. The fight ended with Rance's side winning, and Xavier turning back into a demonic blood soul. The latter was absorbed by Miki, who was the Demon King Little Princess.
During Oda territorial expansion, Rance gets to fight the Houjou House. Nanjou Ran, without knowing, is the body carrier of one of Xavier Apostles, Gigai, which is called Suzaku before he reaches his final form, Ran is forced to use Suzaku to push Oda back.
During one of Houjou youkai hunting sorties, Souun sees how Ran uses Suzaku, and starts researching and discovers the true nature of him, warning Ran not to use Suzaku any longer until he manages to get any way of getting him out of Ran's body, but as both of them take active part in the war, she gets captured by Oda before Souun can make any progress.
Souun falls into depression and leaves Houjou Masako in charge of the Houjou House as he is required to fight youkai Minamoto in a dungeon at Mamushi Oil Field, and traps himself there in order to fight the youkai. Meanwhile Rance finishes the Houjou House off and convinces Ran to fight for Oda, but only as long as they investigate where Souun was, Rance imposes that Ran has to be his lover and he will do it, which she accepts just for Souun's sake. 3G and his youkai contacts manage to track him down to a dungeon at Mamushi Oil Field. Rance, Ran and their party get to the lowest floor and Ran uses Suzaku to finish Minamoto off, but Souun is very heavily wounded and has to be treated in Owari.
A doctor called Keikoku comes to treat Souun wounds in a suspicious way, but no one can realize because Suzume and Chaos discover that Nobunaga is Xavier, and he has been stealing the gourds that contained his remains, growing in power. During the war with the Tenshi Sect, Oda managed to capture Manmaru, one of the three girls of the triplet that Seigan was going to use to seal Xavier, and as Rance is enjoying one of his "love sessions" with her, she asks him to release her in order to be able for Seigan to seal Xavier. Rance accepts and then organizes a force to kill the demon before he grows too strong.
Rance and his party go to the Honnō-ji Temple in order to fight Xavier and his awakened apostles. During the fight, Kentarou kills Rengoku very easily catching him off guard and Rance wounds Xavier heavily, allowing Seigan and the triplet to come inside the room and successfully seal him.
Meanwhile Souun slowly recovers thanks to Keikoku's medical treatment, but when Ran visits him he shows little attention to her and tells her continously to stay with Rance because she is now his girlfriend, making Ran's heart shatter little by little. Keikoku tells Ran to let Souun rest and recover everytime and drives her outside the room.
As the war continues a new superpower grows in the southern part of JAPAN, as the Shimazu House starts offering the remaining southern houses alliance pacts, absorbing said houses territory as theirs, furthermore, they begin seducing female commanders in Oda's army, making them join theirs. As both clans clash swords Keikoku takes Souun with her and dissapears, 3G, Ran and Rance realize that no one of them requested Keikoku's help, as they were thinking the other did it.
Keikoku uses Souun power to start opening Holes to Hell in most of Oda's territory in order to weaken them more, ogres start waging from the holes attacking Oda's territory making their army to split in order to keep fighting against Shimazu and the ogres, slowing the southern campaign.
Eventually Oda manage to stop and close all the holes thanks to the efforts of Ran and her sealing powers, their war against Shimazu goes well and they push Shimazu back to Africa, annihilating their army.
Rance and company discover the true nature of Keikoku and start investigating where she and Souun could be. Keikoku has been using Souun power all along to slow Oda, and she goes deep into a cave at Saitama in order to awake a very powerful youkai called Sekimei, who was sealed by the first generation of Houjou diviners. Every commander in Oda goes to said dungeon and find Keikoku and Souun, accompanied with Sekimei, who has awakened already.
Keikoku reveals all his plans to Oda's party, specially to Ran, who keeps calling Souun's name in order to make him react to her voice, but Souun has been under Keikoku's control all along, so he was not held accountable for his actions. After a tedious fight Rance manages to kill Sekimei, making Keikoku run away without them being able to capture her.
Souun recovers conscience and is informed of all that happened while he was abducted, leaving Ran the choice of going back with Souun or staying with Rance...
After the seventh gourd breaks, Princess Aroma disappears. Unfortunately, when Rance and the others find her, she already passed away. Further more, Nobunaga is assumed to be dead as well. Eventually, Nobunaga's will was found. His will states that Rance will lead the Oda House in his absence, with the condition that Rance will produce an heir with a Japanese woman whom will then lead the House. With an anti-pregnancy spell cast by Sill, Rance used this as an excuse with many women in JAPAN (whom wanted to have a child to become the next heir). Eventually, Keikoku uses incense to render Rance unconcious, and claimed they H-ed and she was pregnant. As a result of this, she forced 3G to pay for costs of the baby. Yamamoto Isoroku discovered that the baby is actually not Rance's, and fought Keikoku. Keikoku ran away after people came to help Isoroku. At first, Yamamoto originally wanted an heir to revive her House, but eventually confessed her love to Rance and that she does not care about having a child to revive the house anymore. Rance was moved by her confession, and ordered Sill to remove the spell. As a result of this, she eventually became pregnant.
Xavier eventually started attacking JAPAN. During these attacks, Kentaro got seriously injured, and Miki turned him into a Demon to save him but felt guilty in turning Kentaro into a "monster" and ran away. Without the Lemons in her possession, Miki's Demon King nature start to surface. Rance eventually finds her to Miki's annoyance, and she was about to freeze him. Isoroku shows up and was about take the hit for Rance, but luckily Miki's human nature came back in time, caring about the baby in Isoroku's womb, so she didn't get frozen. The baby later started to act up, so Isoroku can no longer participate in battles. The war with Xavier goes on, until Xavier loses patience and flew to Oda's castle to directly challenge Rance's group. The battle was tough, but Isoroku shot an arrow right in Xavier/Nobunaga's eye, greatly weakening Xavier. In a raging counterattack, Xavier cut off one of Isoroku's legs. Rance's group were able to defeat Xavier as result of Isoroku's action, and Isoroku was able to survive, but became handicapped due to the assault. She gave birth to a son, but during this time Rance left JAPAN. Approximately ten years later, Rance organizes a massive army to fight against the Demon Realm. JAPAN is led by Ranki, who is Rance and Isoroku's son. Prior to leaving for battle, Ranki was suited up by his mother and sent off by her.
While in a cave in Sado, Uesugi Kenshin finds the Emperor Sword. As a result of this, Amaterasu begins the Emperor Race, which decides who will become the Emperor of Japan. Several of the competitors in the Emperor Race include Xavier (who has the Emperor Hachimaki), Keikoku (who has the Emperor Ring), Nozumi, Baba Shyouen, One Eyed Masamune, and Yuzuhara Yuzumi among others. Most of the competition involved one on one battles between Kenshin and the other potential Emperors. After getting 100 points, Kenshin wins and becomes the Emperor of JAPAN. With the Emperor's Aura, she was able to vassalize opposing houses. Eventually, she used the ability to control Nobunaga, causing Xavier to flee from Nobunaga's body. Xavier escapes to Orochi dungeon, with the hopes of controlling Orochi to destroy the country. In the end, Rance kills Xavier.
When Nobunaga welcomed Rance into the Oda House, many retainers of the Oda House rebelled. Their excuse for rebelling was that he is a foreigner. In reality though, the rebellion was actually contrived by the Ashikaga House. The captains who rebelled were already on Ashikaga's side, and Nobunaga saw this as a good opportunity to flush out all the disloyal retainers in his house. He was willing to do this even though he would lose half of his captains, as he wanted a secure future for Princess Aroma through loyal retainers. Interesting fact: Players may notice that every single enemy captain fought in the intro battle ended up in Ashikaga House, besides Oda Nobuyuki.
Miki is the Demon King known as Little Princess. She resists the demonic nature, but physically she's already the demon. In order to do this, she requires lemons that suppress that nature. She and her childhood friend Kentarou wander from place to place, hiding from demons. One faction of Demons want to kill Miki so the leader of the faction can take over and become the Demon King. The other faction wants Miki to take up the mantle of the Demon King. They happened to come to JAPAN, and stayed with a random house (which happened to be hostile with Oda House). When Kentarou dueled the Seiryou acolyte and was pierced from the back, Miki lost control of herself, causing a great explosion and killing the Seiryou acylote in the process. In an attempt to save Kentarou's life, she gave Kentarou part of her blood, turning Kentarou into a demon. Due to her causing the transformation of Kentarou, she ran away even though Kentarou doesn't blame her for the action. Without the lemons, the demonic nature was getting a better hold of Miki. When Rance finds her, she apparently was very annoyed with him and wanted to freeze him in ice. Sill took the hit instead (due to her and Miki's friendship and out of loyalty to her master Rance). The shock of Sill becoming frozen caused her humanity to take hold a bit, and Kentarou convinced her to have some lemon to further suppress her demonic nature.
Further Note: If you try letting Miki free Sill from the ice, she wouldn't be able to control her powers and end up shattering the whole thing, causing a Game Over.