I. THE CURSE OF THE MISHIMA BLOOD
After the first tournament, Heihachi claims the Zaibatsu for himself and locks Jinpachi underneath the family monastery Hon-Maru where he starves to death. Following this act of downright cruelty, the power-hungry Heihachi being downright annoyed at the continuous weakness shown by his 5-year old son Kazuya decides to throw him into a deep ravine with the proclamation that if Kazuya was indeed his son he would find a way back to him. And Kazuya does so but in doing so he makes a deal with the Devil (Yes, literally) and in exchange inherits the Devil Gene, thereby, cursing the already accursed Mishima Bloodline for good. This gene eventually passes onto Kazuya's son Jin when Kazuya is killed by Heihachi at the end of Tekken 2 by being thrown into a volcano.
When the Fighting God - Ogre resurfaces and kills Jun Kazama, Jin's mother, some 15 years after Tekken 2 Jin goes to Heihachi as per his mother's instructions and trains under him to sought revenge against the entity Ogre unaware that the ruthless Heihachi has his own agenda to fulfill. So when Jin finally defeats Ogre, he is mortally wounded by Heihachi only for the Devil Gene to take over and save his life. Realizing that he requires the Devil Gene as well in order to splice Ogre's DNA into his for immortality, Heihachi strengthens his search for Jin.
It is at the precise moment that the just-stirring Heihachi and Kazuya are attacked by a set of Jacks, mechanical humanoids capable of relentless destruction, where Kazuya betrays Heihachi after fighting alongside him initially before one of the Jack self-explodes. It is assumed Heihachi is dead but just then a King of the Iron Fist Tournament is announced by the Zaibatsu and it is discovered that Jinpachi Mishima, the father of Heihachi, had taken up the mantle of the Zaibatsu leadership. But he is no more human. It seems that an entity signifying the Devil Gene had been the cause behind his return from the dead. (Talk about messed up family melodrama)
The fighting styles contrast in nature majorly as Hwoarang practices a more conventional Taekwondo style against Jin's initial Mishima style Karate to a cleaner form of Karate mixed with Aikido.
Both are trained in the same fighting style combination of Koppo and assassin based Aikido and their nature tend to complement each other. For example, Nina is cold-blooded out and out and bears a deep disgust for all men. Anna on the other hand is more flirtatious and prefers to be seen as a 'femme-fatale.' It is the nature of their relationship that makes their rivalry all that much intriguing
Having put forward the basic essentials one thing that needs to be looked into is the fact that the free flowing hand to hand combat in line with varying styles and characters has given Tekken its own stature to stand upon and it is a game very close to my heart and despite the less than clarified motives for each character, once you start playing it you would likely be one of the many who would be glued to it for hours.
Best of luck with that.
P.S. The Panda and Bear characters do appear out of place but your opinion will definitely shift once you experience their moves. (In the right way that is.)