For whatever reason, Ultros wants to be a villain. Of course, throughout the series, he comes off as a troll.
Ultros: Mwa, ha, ha! I’ll pretend to be Setzer and foil their little plan!
I can’t blame him for aspiring to be something greater than himself. However, being a villain is never a worthy goal. But why is he doing this? He’s carnivorous, so perhaps his main goal is to eat our characters. He’s also lecherous, so maybe his secondary goal is to flirt with women. Or perhaps he wants to assert himself as royalty.
Ultros: Silence! You are in the presence of octopus royalty! A lowborn thug like you could never defeat me!
Well, like most villains, he’s delusional.
But something about Ultros sets him apart from other recurring villains in the series. Like Gilgamesh, he breaks the fourth wall. I think they’re both aware that there’s more to their reality; Gilgamesh routinely makes pop culture references. Ultros, however, predicts the series. Moreso, he’s acting like villains in future Final Fantasy games.
Our first battle with Ultros takes place on the Lethe River. Obviously, he wants to eat them. But why is he in a river? No species of octopi live in fresh water. So then, was eating them a convenient excuse? Perhaps. He’s re-enacting the final battle of Final Fantasy XIII-2. There, Noel and Serah defeat Caius on the beach of Vanhalla. Wouldn’t you know it, he stars in that game. Finally, Ultros is a star! Well, for DLC. He’s not the main villain and is most likely resentful of it.
Our second battle takes place in an Opera house. Already, this is beginning to be unbelievable. Well, it is a fantasy, a Final Fantasy, after all. And octopus can survive on land. But in an Opera House? Who’s battle is he copying here? Ultimecia. The final battle takes place in an exorbitant castle. The Opera House is a stretch, but he doesn’t have much to work with here. Final Fantasy IX was in production at the same time as Final Fantasy VIII. He probably saw Kuja in the Treno Auction house and started forming ideas for another battle. He scrapped the idea when he realized Kuja doesn’t fight a battle there. He wouldn’t fight in the Auction House in Jidoor either.
The third battle takes place in the Esper Cave. Why would an octopus be in a cave? He says he’s trying to steal the Warring Triad, but the reasons are irrelevant. This battle mimics Sephiroth’s in the Northern Crater.
The fourth battle is in the air. THE AIR!!! No, an octopus has no business in the air. Yet he’s copying the final battle with Vayne, who turns into the Undying in the Bahamut Sky Fortress. And surprise surprise, he was in that game too (as a lecherous flan no less).
Why is he doing this? Well, he knows they will be the main villain in their respective games. He decides to beat them to the punch. Think of this as his auditioning role, to prove he’s a capable villain.
Then of course, it could be the other way around. Maybe Sephiroth, Ultimecia, Vayne and Caius are copying him?