Aaron Taylor-Johnson's Quicksilver was one of the MCU's briefest heroes, having started his arc as an enemy of the Avengers in Age Of Ultron, but if Wanda ends up being the cause of the Multiverse crisis in Phase 4, his impact could be felt in an unprecedented way. Sure, a lot of things have happened to Wanda beyond the end of Age Of Ultron, but in the logic of her arc, Pietro's death has always been presented as one of the reasons why some of her fellow Avengers considered Scarlet Witch a threat to herself and to others. It is a deep-rooted part of the experiences that have led her to where she is in WandaVision and to ignore that would be criminal.

Related: How Every MCU Movie So Far Sets Up WandaVision

The question now, of course, is why Wanda drew the line at only really undoing the last major traumatic event in her life, when there were so many of them. Surely bringing back Pietro after his Avengers: Age Of Ultron death would have fit with what her fake reality is trying to achieve? The answer to that may yet be revealed in the show - particularly with rumors that a version of Quicksilver might appear in WandaVision - but it does actually make sense. First, Vision's death robbed Wanda of what WandaVision gives her back - the chance of a normal, happy life - and it's that that leads to her apparent break from reality. And on top of that Pietro becoming another character in her sitcom that she'd have to control to ensure the reality was upheld might well have been a step too far even for Scarlet Witch.

To address the first point, WandaVision is a reminder of everything Wanda lost in her life: her family were killed, including Pietro of course, she was robbed of her own agency by both HYDRA and even the Avengers in the wake of her accidental massacre in Africa and then she lost her heart when Vision died. But it's arguably the latter that's the most important because it caused the house of cards to fall just when Wanda and Vision had found themselves some normalcy. They might have had to meet in secret in Infinity War, but they could be together and they could play at being a normal, happy couple. To show them that chance for happiness before killing off Vision was particularly cruel, but it was a calculated move by the MCU writers to set up WandaVision. It was always going to be the central narrative point here and Quicksilver would have been an initial distraction, particularly as Wanda had been shown as having come to with his death as a hero.

Narratively speaking, it's harder to explain the presence of a brother in the framework Wanda has established for her fake sitcom world. In the typical sitcoms she's based "her Westview" on, a brother character would be a guest character, just as Samantha's mother Endora was in key influence Bewitched. Later sitcoms like Friends would make the "brother as friend" trope more acceptable, so Quicksilver's reappearance might make more sense there. But it still doesn't address the fact that Wanda seemingly had an opportunity to build her perfect world minus every real-life trauma that shaped Scarlet Witch's MCU story and left her brother out. Perhaps that's down to how tied Pietro was to a whole part of Wanda's experience that she simply cannot unpick from the trauma.

Wanda and Pietro were HYDRA lab-rats, put through unimaginable experiments that killed every other test subject and then turned into Avengers-hating weapons. The leap from there to their defection from Ultron to the team was short, so Wanda's memories of her brother as an adult would be deeply tied to those trauma points. Just as she's cut out her past as an Avenger and as Scarlet Witch even, Wanda may have had to suppress her brother's memory entirely. Sure, he could be logged away as a childhood memory, but WandaVision has already established that proximity to the truth is the quickest way to destabilize Westview's precarious fake reality. It's a difficult one to accept on the surface, with Vision's Infinity War death undone, but it perhaps speaks to Wanda's enduring sense of self-preservation that she's not brought Pietro back as another possible threat to the bubble she's seemingly conjured.

Next: WandaVision: How Scarlet Witch Could Resurrect Quicksilver In The MCU