When the first P4 Arena released, it seemed crazy obvious that Nyarlathotep, the big bad from Persona 1/2 was back! It made sense.
- The tactics this mysterious entity used in Kanji's story mode lined up exactly with Nyarly's shenanigans in P2, and dialogue before/after the final battle in Naoto's and the P3 characters' story alluded to this too.
- The Malevolent entity needed to team up with a human to fuck with people, much like Nyarlathotep needed to in P1-2.
- At the end of P2 EP, Nyarlathotep promised he would return and seek vengeance on humanity.
- It was implied that members of P1 and 2 joined Mitsuru's anti-shadow force.
- The announcer of the battles is the same voice actor (in English at least) who did Nyarlathotep in P2 Innocent Sin PSP remake.
It was hype as fuck, not only to fans of the Persona 1 and the 2 duology, but to people who never played it and only had passing knowledge of those games (like myself at the time). It made the Arena games truly feel like a celebration of the entire series, even though there were no P1 or 2 characters playable! And it got some people to finally play through the old games before the Persona 4 Arena sequel released (in my case, just the Persona 2 duology)...
...Then Ultimax released and it turned out to be some random evil Japanese god pulled out of nowhere who was never alluded to and no one ever could have predicted prior to the reveal in Ultimax. 🙄