Emperor Evulz remembers how he killed Mana's boyfriend. Mana doesn't.
Emperor Evulz only remembers killing Mana's boyfriend, and no one else.
"I beat last-rate bad guys all the time, you can't expect me to remember all of them." See Avenging the Villain.
Mana runs into the heroes and thanks them for saving her village the last time they met. The heroes are dumbfounded, as they forgot all about it.
Mana savvily waits until she's delivered the lethal blow to Evulz before informing the Almost Dead Guy that she killed him for wiping out her hometown, and quotes the trope, expecting him not to remember it. Turns out he was Necessarily Evil, and he remembers destroying her hometown vividly — doing it saved exponentially more lives for some reason. Perhaps it was even His Greatest Failure — if he'd been better at his job, he wouldn't have had to sacrifice those lives to save others.
When Mana tells Evulz her boyfriend's name, Evulz suddenly starts a long rant. He keeps praising the man's bravery and affirms that he was a Worthy Opponent, how the victory over Mana's boyfriend is Evulz's greatest achievement, and how he feels his life is so empty without his rival. Mana is dumbfounded, for she never knew he was that great.
Turns out he burned down her home town because years ago she burnt down his home town...and she doesn't remember.
Mana, a Damsel in Distress, has been kidnapped and almost killed so many times that she's forgotten who Emperor Evulz is and all the times he tried to kidnap her.
When confronted, Evulz doesn't remember wiping out Mana's hometown... until he recognizes the scar he put on her left cheek.
Evulz doesn't remember most of the places he razes, but Mana's hometown was special... After all, he grew up there too.
Mana realizes that Evulz doesn't remember the incident because it really wasn't him; Mana was thinking of someone else all along.
Turns out he was remembering scarring someone else's left cheek while burning down their hometown.
He remembers, but only vaguely, and still doesn't give a damn. He forgets it about it later.
Upon learning that Emperor Evulz doesn't remember the wrong he committed against Mana, Mana begins to explain in detail what the wrong was in an attempt to jog Evulz's memory. Evulz still doesn't remember.
Evulz doesn't remember wiping out Mana's hometown, but he does remember something trivial that is vaguely connected to wiping out Mana's hometown, like he got a lot of blood on his shirt and wanted to wash it off, but his washing machine at home was busted, so he had to go to a drycleaner, but he left his credit card at home so he had to pay with cash and was five dollars short...
Mana takes Evulz back to her hometown, which after years of rebuilding has finally got back on its feet, and encourages him to raze it again in an attempt to jog his memory.
Zig Zagged: Emperor Evulz wiped out Mana's homeland. When the heroes confront him, he recognizes them and starts bragging about how he destroyed it... but then they realize he's not talking about Mana. Turns out he destroyed Alice and Bob's hometown, too — something they hadn't known about yet.
Emperor Evulz knows exactly who Mana is and why she has a grudge against him.
Emperor Evulz doesn't know who Mana is, but the instant she mentions the town, he puts two and two together.
Enforced: The editors of the story wanted to make Emperor Evulz even more horrible and evil than was originally planned to make the Anti-Hero Mana look more like a good guy. To compensate for the Emperor's escalation, the writer doesn't have him remember the now comparatively tiny crime of destroying Mana's hometown.
"My dear, you can't expect me to remember every pathetic little village I've wiped out in my time."
"It was Thursday, you sick son of a bitch!" "Was it, now? Well, I'm sure you remember it better than I do."
Invoked: Emperor Evulz pretends not to remember just to enrage the heroes, expecting their anger to blind them and lead to them making critical mistakes in the following fight.
Exploited: The heroes take advantage of Evulz's terrible memory for atrocities so they can pose as a different victim of his, evading notice.
Before he escaped, Mana managed to gave Emperor Evulz a distinctive scar on his face precisely so that he wouldn't forget her or what he did to her.
Evulz keeps a running journal of his misdeeds
Evulz has a cell built, with the name of each town he razed carved into the walls. Once a week, he visits that cell, and has himself whipped while he chants their names.
Discussed: "Mana, this psycho commits at least two world-shattering atrocities every day before lunch. You really think that killing your father is going to stand out in his mind?"
Conversed: "I'm waiting for a scene where somebody calls Evulz on the phone and gets the message: 'Please leave your name, number, and relation to people I've killed, and I'll get back to you shortly.'"
Implied: The hero doesn't say much, but he obviously holds a grudge against Evulz, one which Evulz is honestly surprised to learn of.
Evulz doesn't remember who Mana is until she reminds him in specific detail. Then, he asks her the names of all the members of his Legion of Doom that she's killed, and where they live, and if they have families. He then shows her a memorial he's built for the dead soldiers, and a gigantic stack of letters he was writing to their families.
Alternatively, Evulz's multiple acts of reckless Kick the Dog moments pile up throughout his career, causing the rise of one revenge-seeking hero after another. They team up to hunt down Evulz en masse and defeat him. Evulz doesn't know how to deal with this; he doesn't remember any of them.
Emperor Evulz and Mana's allies call her out for focusing one on a single event rather than the sum total of his travesties.
Reconstructed: Evulz doesn't recognise the name of the town, but finds out where it is and unapologetically explains that it was part of a scorched earth campaign to keep his enemies from sheltering there. Of course, a man with World Domination on his to-do list can't fight a personal duel with every kid whose father he may have killed, but have you met The Dragon? You kids play nicely now.
Played For Laughs:
"Oh yeah! It bothered me that the body count was one short. Well, time to fix that mistake."
"Oh, it's all coming back to me, I went to Subway before that."
"Still not ringing any bells... Maybe you should talk to my secretary? She usually knows when I schedule my massacres."
Evulz pulls out his diary. "Let's see...March 4, March 4...ah, here it is! Now I remember! 'The smoke rising from the burning buildings looked particularly striking against the cloudless blue sky. As I rode into the sunset I spotted a solitary child glaring balefully from the rubble, doubtless plotting her eventual revenge.' Good times, good times."
Played For Drama: Mana takes her revenge on Evulz, but her victory feels hollow to her, since Evulz didn't completely understand what he had done to deserve it.