Initially Amnesia: Memories seems like a strange experiment with four different realities where the player is a particular girl who wakes up with amnesia and has to find out who she was; simply variations on a theme.
But after playing the different storylines, and particularly if the player hits a few bad endings, it becomes apparent that there is more going on.
A mysterious character called Ukyo shows up in all routes, though he's not always named and does not always have much interaction with the protagonist. However, some of his conversations make it clear that they have met before before and allude to him having existed in multiple realities.
It's only after getting the good endings for all four initial love interests that the Joker world unlocks, and there's a good reason for that. The effectiveness of Ukyo's storyline depends on the player knowing what has happened in other worlds, even if the protagonist has less of a clue.
Unlike most storylines, Ukyo does not start out already in a relationship with the protagonist and cryptically comes and goes, leaving warnings about danger. His warnings are not to be disregarded. Doing so quickly results in the protagonist getting killed, giving him more bad endings on his route than any other love interest.
Oddly enough, there is one bad ending where Toma goes psycho and locks the protagonist in a cage to protect her again. (Really? Did we have to revisit that?)
Ukyo's route also dispenses with a lot of the introductory stuff that the other storylines do. Though there are still some elements of the protagonist trying to find her memories, depending on player choices they can be oddly sparse in this world. The game also throws its entire cast of characters in this route, giving them all a reasonable amount of time in the spotlight, which works best with the player already knowing who they are and what their personalities are like.
In a way it feels like a last hurrah, because the game knows this is the last route and the player's last chance to spend time with these characters. I admit I did particularly like the shaved ice speed eating contest between all the love interests, culminating in Shin getting a case of head freeze due to trying to win. He might act like he doesn't care, but he sure does.
In fact, if the player doesn't make a point to seek out Ukyo, there are a ton of alternate events on this route, more than any other, where the protagonist and the rest of the cast can just be friends. Ukyo's route is one of my favorite realities because of it. Everyone knows everyone else, and even the Ikki fan club president (who's a bitch on all other routes) is a decent person.
Ukyo's route is a little frustrating to play though, because we know he has the key to everything (or almost everything), but he refuses to get close to the protagonist, claiming that he's dangerous, even as he's giving whatever information he can to protect her. His behavior is maddening, especially since the protagonist can barely remember anything about him.
When he visits her in her apartment, he handcuffs himself to the table and gives her pepper spray, a stun gun, and a safety whistle to use in case he gets out of control, which is comical in how seriously he wants her to be able to defend herself against him.
It's not without reason, as the player has seen Ukyo in other worlds and is aware of his split personality. Depending on player choices, his more malicious persona emerges from time to time and that one is clearly not the protagonist's friend. One side of him wishes the protagonist well, the other wishes her harm (and even kills her in some endings).
As a result, the pacing of his story leaves something to be desired, as Ukyo's cards are played far too close to his chest, but what eventually unfolds is a hopelessly romantic story. I won't say that Ukyo's is my favorite route, but it's the only one I cried over.
Ukyo's world is not the "original" world, but it's the one that the Ukyo we know originated from, and Amnesia is not a story of alternate realities so much as many realities.
Joker world Ukyo lost the protagonist in an accident on August 1st, with her eventually succumbing to her injuries on August 25th. His grief and his wish for her to survive drew the attention of Nhil, a god from between worlds who is also Orion's master. Nhil gains power and sustains his own existence from granted wishes, so he wanted to grant Ukyo's wish, for Ukyo to witness a world where she survives past August 25th.
Nhil can't revive the dead, so he merged with Ukyo, making him semi-immortal, and the two of them went reality hopping to find a world where Ukyo and the protagonist could live together beyond August 25th. But they found that he didn't exist in any other reality with her, and if he tried to join a reality where he didn't exist, the world would realize there was a foreign influence and kill him to correct itself by the day of her death (preventing him from seeing whether or not she lived).
These were some of the worlds that the player previously played through. Ukyo began to give up hope, always traveling to a world, always dying by August 25th, but still wanting to see the woman he loved even if she found happiness with someone else. His despair gave birth to his split personality, the part of him that resents dying just so he can watch her live. (And most of Ukyo's deaths are pretty horrible. When the world corrects itself, it's not nice.) His other personality learns that if he kills the protagonist in a given world by August 25th he gets to live in her place, though the sane part of him always departs for another world after recovering himself.
Orion's accident, where he bumps into the protagonist in the prologue, seems to have been an unconscious bid on his part to fulfill Nhil's wishes and find the woman that Ukyo was searching for, which is how the whole amnesia thing starts, even though it's not directly related to what Nhil and Ukyo are doing.
Finally, Nhil decided to make one last gamble and used the remainder of his power to rewind time in the original Joker world to give Ukyo a chance to save the original protagonist that he loved, and he resets the clock to the start of July, a month before the protagonist's death, so Ukyo can prepare. Because this also rewinds past the day they started dating, there are no memories of them dating for her to recover (though she had met Ukyo originally in March, so she does recover the memory of having seen him).
Because Ukyo is aware of the protagonist's eventual fate in Joker world and his own split personality, he behaves differently and keeps his distance from her so he doesn't accidentally kill her before the 25th. Ironically on his part, the only way to get the good ending is to ignore all of Ukyo's warnings about staying away from him and become as close as possible.
Though he does save her from the initial accident on August 1st (assuming the player listens to his warning not to go somewhere), the world spends a crazy amount of time between the 1st and 25th trying to kill the protagonist and correct the fate that didn't happen. It's only because Orion is merged with her that the world doesn't seem to be able to pinpoint her well enough to close in for a complete kill.
The final day of Ukyo's route is pretty frantic, because by then the protagonist has pieced together that Ukyo intends to die in her place, and the world is going haywire with freak weather, mechanical malfunctions, a fire, and more to kill her. And if that wasn't enough, Ukyo's other personality tries to murder her in the final minutes before the clock strikes midnight.
I wasn't really sure what was going to happen.
But Ukyo stabs himself to prevent his other personality from killing her, giving her a reprieve long enough for the clock to roll over to midnight. Knowing that she has survived to August 26th, Ukyo's wish is granted.
A good chunk of the backstory comes out at this point (they really saved everything up until the end!) including that Ukyo has killed her in other realities, and the player is asked whether or not they forgive him.
For me, it was a no brainer. If you want tragic, suffering romance, Ukyo is the poster boy for it. It's an uneven route and filled with a lot of instant death pitfalls, but if one doesn't mind an info dump for a reward and knowledge about how this whole scenario started, it's a fitting capstone to Amnesia as a whole.