Spoilers Off as per wiki policy
- Yukari Yakumo's backstory, derived from the common Fanon that she's who Maribel would end up becoming at the end of her timeline. She developed a close relationship with fellow member of the Secret Sealing Club, Renko Usami. Despite how close they were, it all came to an end when Mary had to eventually become Yukari, losing her closest friend and companion Renko through gruesome means in the process.
- By the latter half of "Necrofantasia", we're given a series of sepia CG's showcasing the happy memories of Renko and Maribel, with the last two depicting Renko's bloody end and Maribel's descent into despair, surrounded by many eyes that would be a staple aspect of Yukari. When it cuts back to game-play, Yukari is singing much more somberly, no longer sitting on one of her gaps, waving her parasol around or sending trains after BF, just passionately singing the rest of the song all the way to the end. And possibly unbeknownst to her, she has someone singing alongside her, and who else would that be but the ghost of her lost partner, Renko Usami.
- Yukari's support of Boyfriend and Girlfriend's relationship, while heartfelt, shows Yukari's concern over Girlfriend's parallels with herself, if her comparison to "another young couple who's love was cut short" is any indication. All Yukari can do is advises Girlfriend to cherish and love Boyfriend, along with a warning to keep her demonic power under control, lest history repeats itself. And chances are, history has already repeated itself, if the presence of "Kiki Setsuko" is any indication...
- The omake alongside "Necrofantasia", detailing an event in the past involving Renko and Maribel having a drinking session. Renko, likely in a drunken thought, decides to ask Maribel about marriage, with the latter responding back with more flirting after getting flustered. While generally a cute moment between the two, things sour quickly when Yukari reawakens from the memory to grab a black box with an engagement ring inside. While Yukari briefly considered where she'd be if the proposal happened, she ultimately tells herself off:Yukari: It’s no use dwelling on what if's. Renko... was fated to die. If she stayed alive then all that I care about would never exist. It had to be this way...
- The "Border of Life" omake details an event between Maribel and Chouka (Yuyuko). The former having just moved on from Renko to have Chouka as her lover, putting Maribel at peace for now. But like in canon, Yuyuko is fated to be Driven to Suicide and become a ghost without her prior memories, which Chouka is well aware of. As Chouka discusses her potential fate to Maribel, she asks them treat her spirit self as a different person, being effectively told to let go so soon after already losing a prior loved one. All Maribel can do is cry herself out as Chouka comforts her.
- Kiki Setsuko, and all the depressing implications that come with what's presented about her:
- She's an older, more powerful Girlfriend who went through a similar situation to Yukari: getting time-displaced and losing Boyfriend to unexplained circumstances, followed by living the rest of her life in Gensokyo while holding on to the memory of him, as indicated by her Tragic Keepsake: a well-worn red cap that used to belong to him.
- In her appearance for the "Charming Domination" omake, she's the culprit of the "Missing Sounds" incident, having stolen all sounds across Gensokyo to fuel her "Party of Remembrance", implied to be for her long dead lover. Needless to say, she's not happy about having to cut the remembrance party short because some miko told her so, indicated by her punching the ground with enough force to crack it.
- Stage 6, the conclusion for the story of Phantastical Boundaries, with a Downer Ending that those who follow the story will dreadfully see coming.
- It all starts off rather wholesome, with the two couples, Renko and Maribel plus BF and GF, going on a space trip for their honeymoon, with BF and Maribel planning to pop the question before their lovers. What makes this particularly tragic is the elements — the small black box carrying a ring sounding like what Yukari owns as a Tragic Keepsake, GF's new ensemble she's sported since the Time Skip resembling Kiki Setsuko — that alludes to what will inevitably happen.
- Right when things were the happiest between Renko and Maribel as they share a kiss, the latter has her mind suddenly invaded by an entity, Yukari herself, who says she should pay the price for her power over boundaries. Even as Maribel tries to defy them, they get afflicted with some form of corruption that overtakes them, with their first act in this state to murder her own lover — and only after the fact does she realize what she has done.Renko: M-Maribel... It's not... your fault...
Maribel: Renko? No please... Renko... RENKOOOOOOOOOOO!!! - Eventually, BF and GF gets caught in the crossfire. As the corruption overtakes Maribel once again, she prepares to take BF as her food source, with GF attempting to Mercy Kill her to save them... an attempt that "Maribel" uses to portal GF's hand through a gap to kill BF instead, sending GF into her own despair.Girlfriend: BOYFRIEND!!!! Nonononono! Please Boyfriend, don't die on me... please! I didn't mean to!
Boyfriend: I... love you... - Yukari, after all the above, discusses with Kiki that she had to manipulate herself into killing Renko, as fate had decided that this should happen regardless of how she feels, alluding to many alternate timelines where she tried to save her, only to be met with failure.
