Dandadan is a world teeming with otherworldly beings, from aliens to supernatural yokai. Among them is Reiko Kashima, the infamous Slit-Mouthed Woman, a yokai who stood as one of the main villains Momo had to battle against during the Space Globalists arc.
Reiko was behind the supernatural events in Kamikoshi City and was ultimately banished by Momo to another dimension. Since then, she vowed to get her revenge on Momo and carried a deep hatred for her. However, after witnessing Momo's relationship with Okarun, she let go of her hatred and decided to seek love for herself. Some fans believe she may return to the series in the upcoming arc.
Disclaimer: This article is a speculative theory and reflects the writer's opinion.
Why Reiko Kashima might be the yokai to return soon in Dandadan

Reiko Kashima is the strongest yokai in Dandadan and could make a major return in the next arc. So far, the story has seen Okarun return the abilities he received from Turbo Granny after retrieving the second Kintama. Gaining back her abilities, Granny decided to leave, leaving Okarun without her powers. Since then, fans have been wondering when Granny will return to the series
However, the upcoming arc might require Reiko to make a return more than Turbo Granny. Momo is currently much larger in the series after being cursed by a fairy tale card and how this curse can be broken has still not been revealed. Even Seiko doesn't know if they'll be able to get her back to normal size. Thus, there can only be two other entities capable of helping Momo, Turbo Granny and Reiko.

A fan theory suggests that Momo getting back to normal could involve her growing emotionally and confessing her feelings to Okarun. In the past, Reiko had warned Momo to confess her feelings or she might take Okarun from her, so it might be a possibility that Reiko could play a role in Momo getting back to normal size.
Reiko is a fascinating character that fans would love to see return to the series and cause chaos again but it is difficult to envision a scenario where she might be inclined to help Momo, seeing that she still passionately hates Momo. However, it's interesting to speculate a scenario where Reiko falls for Okarun and Momo has to confess her feelings to him before Reiko can which helps turn Momo to her normal size.

It is true that Reiko has gone through a lot of character development and is looking for a suitable partner currently and it might be this pursuit that ties her fate together with the protagonists. This is an interesting speculation as Reiko has become a fan-favorite character and it would be interesting to see her return to Dandadan.
However, a better alternative is that Turbo Granny is currently searching for a cure for Momo, which is why she decided to leave. She is an extremely beloved character and is bound to return to the series soon. Thus, she will likely be the one to help out Momo seeing she has grown quite fond of her and Okarun.
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