's story mode and character episodes have confirmed a popular fan theory that Reina is the illegitimate daughter of the former face of the series - Heihachi Mishima
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Prior to Tekken 8's launch, Reina's background has been treated as a bit of a mystery, , "surrounded by mystery as the only known fact about her is that she is a student at the Mishima Polytechnical School". This mystery is one that's revealed as part of Tekken 8's story mode and Reina's character episode.
In the tenth chapter of Tekken 8's story mode, Reina is seen going up against Nina Williams, where she questions who she really is. Nina doesn't let us hear the answer, but it makes it clear that she's related to someone we already know. Considering Reina knows Mishima style and has a lot of the same moves as Heihachi, the answer has always been pretty obvious, but it's then confirmed in her arcade mode run that is unlocked after beating the main story.
Katsuhiro Harada has previously confirmed that ꦬHeihachi has multiple illegitimate childre🦩n, which means there might be more fighters like Reina out there waiting to be discovered.
As soon as you start up the character episode, it's instantly revealed who Reina really is, as she's described as being the "secret daughter of Heihachi Mishima". It then goes on to say that she has a lot of respect for her father and that she's out to get Kazuya to prove his worth, seemingly not aware that he died at the hands of Kazuya in Tekken 7.
Reina's character episode ending has her imagining taking Heihachi's place after defeating Kazuya, before quickly showing her finishing her daydreaming and say that she was "kidding" before going to take on take part in the tournament's final match. It's not really clear what her motives with that in mind, but she does seem to be on Jin and the rest of the heroes' side.

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