How different they are from him and her, their identities, personalities, hobbies, people they come in contact with, their surrounding environment, and... Their endings.
Logically speaking, they shouldn't have crossed paths. Even if they did, it was just a casual encounter, passing each other by and never seeing each other again.
But Orihara Izaya pursues excitement, while Karizawa Erika likes the unusual.
Therefore, these two people, who can be considered completely unrelated, have an extraordinary relationship.
This is serious. No kidding.
[This story is purely fictitious]