Season 1

Jack was born in a family of warriors. At first, he refused to become a warrior like his father. But then he got bullied seriously at his school. With that, he decided to learn to defend himself by taking swordsman training. Even after he finished those trainings, the bullies were not afraid of him and continued to treat him badly. Gaster got to know him and defended him from the attacks of the pesky bullies. Being thankful of what Gaster did, Jack became his loyal assistant.

Jazzy was one of the harems of Zenny, the evil, bad-tempered king of the underground (He needed the lockets of his seven harems to make himself invincible, like what Flowey did in Undertale). She once had lots of problems with Zenny, causing her to flee away from the relationship in tears. She then met Jack, and she fell in love with him shortly after. They had lots of great and lovely memories together. However, Jazzy soon realized that she's still in love with Zenny, escaping from her relationship with Jack. Jack knew that Jazzy cannot get true love from Zenny, who abused her and even tried to get her locket, so he tried his best to protect and convince her, but she didn't listen to him. Depressed and heartbroken, Jack became an enemy of Zenny.

Meanwhile, Jack and Gaster just finished creating their masterpiece, the trait of Confidence. Jack tried to combine his locket (Originally with Determination) with the new trait, causing him to lose control of his emotions. Once he was back to his normal self, he realized that his hands were stained with Zenny's blood...he murdered Zenny accidentally.

Upon knowing the death of her love, Jazzy surrounded herself with hate, and was determined to end the life of Jack...an eye for an eye.

Soon, Jack found Frisk in the forests. Frisk was trying to kill herself, because her locket was accidentally filled with hate and it was so strong that she was unable to control it. Seeing this, Jack took her back to his laboratory and gave her some hate inhibitors, which reduced the influence of her hate for a short period of time.

During her stay in the lab, Frisk was taken care by Jack, since she was a bit weak after the injection of the inhibitor. She got to know the things between Jack and Jazzy, so she decided to help Jack fight against Jazzy. After a few weeks, Jack and Frisk fell in love with each other.

Frisk then tried to remove her hate trait by giving her knife, which was filled with hate, to Jack. Jack was determined to help, but he soon got affected by hate, too. They helped each other to get rid of the evil trait between them and they loved each other more than ever.

The days have passed, and what beholds, is the battle of judgement and love: The fight between Jazzy, Jack and Frisk.

A few days later, Jack and Frisk fought with Jazzy in the judgement hall. At the middle of the battle, Jack almost got killed and fainted. Thinking that Jack was dead, Frisk surrounded herself with despair. To her surprise, Jack woke up in a few hours and landed a final blow on Jazzy, killing her. Thus, Jack and Frisk came out of the battle victorious.