The biggest retcon was in Tekken 4 though. The Devil "Gene" thing. Originally, Kazuya, when getting chucked off that cliff sold his soul to get revenge on Heihachi right? Okay so this would imply that the series has supernatural stuff in it. When Kazuya "dies" in Tekken 2, the Devil spirit then moves on to Jin.
But then it's revealed that the Devil power is a bloodline thing after that(with eventually Kazumi being shown to have those powers too). Except Jinpachi, who's actually a pissed off ghost.
One thing led to another and somehow we got stuck with Lars and Alisa.
In Tekken 1, the plot is straight forward, Kazuya was thrown off a cliff by his dad, Heihachi, when he was a kid because he was a weakling, but he survived the fall, and spent his life training to become strong so he could take revenge.
In Tekken 2, however, we learn the Kazuya survived his fall only because he made a literal deal with the devil on death's doorstep. It turns him evil, but luckily Jun has like, anti-devil powers so whenever she's near him, he's normal.
In Tekken 3, we jump forward in time a bit and learn that Kazuya has died, but not before he and Jun had a kid, Jin. It turns out, Kazuya passed on his curse to Jin, but because Jin was always near his mother, there was no sign of it until he was an adult. So far so good.
In Tekken 4, things start to get extra wonky. We learn that Kazuya and Jin possess the Devil Gene, a literal genetic predisposition to becoming the devil. It wasn't a deal with the devil that allowed Kazuya to survive being thrown from a cliff, but an actual innate characteristic of his own DNA. Heihachi didn't throw Kazuya off a cliff because he was weak, but instead did it specifically to awaken Kazuya's latent powers. Unlike his son and grandson, however, Heihachi doesn't possess the Devil Gene and therefore has no access to those latent powers. This causes drama. Oh, and also Kazuya is still alive.
In Tekken 5, we're introduced to Jinpachi Mishima, Heihachi's dad, who Heihachi literally imprisoned under the family home because Heihachi is a **** to family members. After he dies of starvation he unlocks his own Devil Gene and becomes a raging monster. We're also introduced to Asuka, Jin's cousin, though neither of them know they're related. Asuka also possess Jun's latent anti-devil powers, though she's unaware of it. Jin, meanwhile, has gone full crazy demon by this point, unable to control his power the way his father can.
In Tekken 6, Jin has taken over the Mishima Zaibatasu and intentionally starts a world war. He's gone full villain, right? Wrong. He's actually still a good guy, and the world war was a huge gambit to awaken Azazel, an ancient evil being who also happens to be the origin of the Devil Gene, somehow, and destroying him will apparently end the curse. Oh, and also it turns out Jinpachi
doesn't possess the Devil Gene, he just turned into a powerful monster for completely unrelated reasons. And Heihachi had an illegitimate son with a Swedish woman.
Finally, there's Tekken 7, where we're finally introduced to Kazuya's mother Kazumi. Who also has the Devil Gene. See, she's the origin of the Devil Gene in the Mishima family, and she specifically married into the Mishima family to ruin the Mishima bloodline, for... reasons. Oh, and also killing Azazel accomplished nothing whatsoever in ending the curse and Jin is actually a bad guy after all, I think?