WARNING: The following contains spoilers for Mars Red Episode 7, "The Letter," now streaming on Funimation.
Episode 7 of Mars Red is mostly a flashback to the events that turned Misaki, Nakajima's daughter and Maeda's fiancé, into a vampire and which eventually led to her death in Episode 1. However, the episode is also peppered with the much more down-to-earth backstory of Shutaru, the newbie vampire of Code Zero, and Aoi, the extroverted journalist who keeps running into every bloodsucker in Tokyo.
Previous episodes showed that Aoi knew Shutaru; in fact, she mentions him to almost anyone, revealing that he enlisted, was sent to Siberia and apparently died -- although she doesn't believe that he's actually dead. Aoi's conviction is not based on any facts or rumors, but on her firm belief that Shutaru would never break a promise, and he promised her that whatever happened he would never die. He didn't entirely break it, either: Shutaru is undead, still enlisted and perfectly hidden by the Japanese government.
The main scene of Episode 7 takes place on a snowy train station, as Shutaru prepares to say his goodbyes to Aoi and her family. Their conversation reveals a bit more about both of them: Shutaru and Aoi grew up in the same town, and when his parents lost their store, he went to live with her family. While he was there, Aoi's older brother passed away. Aoi kept her pain locked up inside, refusing to cry even during the mourning period, something that Shutaru admired because he considered himself a "crybaby" -- one of his reasons for enlisting was to try and become tougher.
Not that anyone close to Shutaru wanted that for him -- he was treated like a member of the family in Aoi's home and having experienced the loss of their son and brother, the last thing they wanted was for Shutaru to go off to war. Aoi keeps insisting that he can stay and that he can always come back, even as she adjusts his military hat and oversized uniform and jokes that the real reason he's leaving is that he "was annoyed that people considered them an engaged couple." However, Aoi's last contact with Shutaru is not a kiss, as one would expect from a young couple, but a sharp kick to his shin right before she makes him promise that he won't die in Siberia.
What's interesting about Aoi and Shutaru's dynamic is that it's so slice-of-life that it's impossible not to root for these two. Most of the other characters in the show have suffered through a tragic love story, from Maeda and Misaki's doomed romance, Yamagami's last goodbyes to his widow and Suwa's intense connection with a young sex worker, abruptly cut short by the outbreak of vampirism in the Red Light District. Mars Red is, so far, structured like a tragedy and every romantic subplot so far has reflected this, but after spending so much time with the plucky Shutaru and Aoi, the death of one or both of them would be a hard pill to swallow.
Aoi and Shutaru are the characters who have changed the most since their flashback. Shutaru kept his promise, and in "not dying" he also became stronger than he ever thought he would be without losing a gram of his natural kindness. In fact, his clear delight in his vampire powers could be traced back to his desire to become worthy of Aoi's strong-willed family.
And, in turn, Aoi has left behind her role as a dutiful daughter and is applying her determination and optimism to uncovering "the truth," whatever that might be and regardless of her preconceived notions. She walks into "Salomé" thinking it's going to be boring, and finds it delightful; she pursues the scattered clues hinting that vampires exist, even dragging her worn-out editor, and she never gives up on Shutaru's unlikely survival in Siberia.
Mars Red is building up to their reunion, playing on the idea that he's a vampire and she's mortal and setting up one ominous romance after another, but it is likely that where their predecessors failed, Shutaru and Aoi will succeed; the balance of the story and the glimmer of hope that makes every tragedy shine that much brighter demands it.