RedHood123 suggested I decide the boss, and who battles the boss. So, here you go.

"Cat-Bat-Hat?" RedHood123 asked. "Is that the name of your persona?"

"That's right." DiamondPickle994 replied. "Part cat, part bat, part hat."

"Ah." RH123 replied. "Anyways, my persona has eight arms, with one different type of gun in each. It has the name: Peacekeeper."

"Nice." DP994 replied. "And that ten gallon hat is a nice touch. On another note, we need to find out whose been causing all this."

"Someone page me?" A voice asked.

Hamister appeared before everyone as fast as the wind.

"You're the ones who went against my army." He informed.

"And you're the one who caused this apocalypse!" RH123 replied.

"Where else shall my minions help in making this place my hamster kingdom?" Hamister replied. "And I see you two have personas now."

"What's it to you?!" DP994 retorted.

"I'll show you what's it to me!" Hamister replied. "Minions! Go forth and destroy them!"

An army of mutant hamster minions rushed at the authors. The two authors summoned their personas.

"Cat-Bat-Hat!" DP994 exclaimed.

"Peacekeeper!" RH123 exclaimed.

While DP994's persona used physical damage, RH123's persona used gun damage.

"Pathetic minions!" Hamister snarled. "Must I do everything myself?!"

Hamister drew his twin knives.

"Feel my wrath!" Hamister exclaimed, summoning a rainstorm of knives.

"Guard!" DP994 exclaimed.

The personas of the authors protected them from the knife rain.

"Cat-Bat-Hat!" DP994 exclaimed. "Attack Hamister!"

"Peacekeeper!" RH123 exclaimed. "Attack Hamister!"

The two personas struck Hamister.

"It'll take more than that, pathetic humans!" Hamister retorted, attacking DP994 in the blink of an eye.

"That damn hamster!" DP994 exclaimed.

Suddenly, a volleyball whacked Hamister on the face.

"Who did that?!" He demanded.

To the surprise of the authors, the one who launched the volleyball was Suguru Kamoshida, who was out of prison.

"I was released early for good behaviour." He explained. "And I'm a registered sex offender, and I've been blacklisted from Sujin. But now, I'm making amends for my actions!"

"Gah! I must return with the others!" Hamister exclaimed. "Do not think that this is over!"

Hamister disappeared.

"Why did you help us?" RH123 asked.

"After I've been released from jail, I vowed to make amends for every bad thing I've done in my life." Kamoshida explained. "I shall aid you on your journey."

Bet you didn't see that coming.