Warning: contains major spoilers for Demon Days: Mariko #1!
Black Widow made her film debut in Iron Man 2, where she successfully went undercover for quite some time within Stark Industries. In Demon Days: Mariko #1, Black Widow is also undercover, though she doesn't do nearly as well with her assignment. In fact, the length of time she spends undercover in Demon Days puts Iron Man 2 to shame.
Natasha Romanoff is one of Marvel's best-known spies. She has had a huge rise in popularity since being included in the MCU as an Avenger. In Iron Man 2 she was placed undercover by Nick Fury and S.H.I.E.L.D. to keep an eye on Tony Stark and his condition. She successfully keeps her cover until Fury reveals himself - and her true role - to Tony. At no point is she compromised in her mission.
Contrarily, in Demon Days: Mariko #1, Black Widow finds herself making the most basic of mistakes - largely due to her lengthy assignment. Created by Zack Davisson, Peach Momoko, and Ariana Maher, this comic gives the spy the name of Kuroki and places her in Japan at Mt. Kirisaki. She is supposed to deliver Mariko - the girl she watches over - to her employer. There's just one major problem: Black Widow can't bring herself to do it.
Despite having a rough beginning, Black Widow has proven herself to be a hero who uses her spy training for good. Even in newer comics that take different approaches with characters, like this one, Black Widow's heroic tendencies shine through. Although she has an Oni for an employer, she knows she has made a vital mistake: she became attached to her target. It is rare that Natasha can't complete a mission, but this iteration of the character has spent so much time close to Mariko that she has a bond with her. This pushes her to break the number one rule of spies - don't form attachments.
Although Black Widow is able to prevent herself from forming an attachment to Tony and Pepper, it is less avoidable in her new circumstances. For one thing, Tony is rather insufferable and he isn't an endearing child - even though he sure likes to act like a child sometimes. For another, Kuroki was undercover much longer than her Iron Man 2 counterpart. By comparison, her assignment with Tony was laughably short. That should be one of the easiest assignments she has ever had - particularly when juxtaposed with this one. It's hard enough spending years undercover, much less when the target is a young child who grows up under Black Widow’s tutelage.
Even though Black Widow fails her assignment in this instance, it proves more positives than negatives for her character. For starters, it accentuates that even with her training, she isn't heartless. She may be cold and calculating at times, but that doesn't mean she'll turn over a youth to be killed for their blood. Additionally, the fact that neither Mariko nor her adoptive grandmother suspected anything despite her being present for years speaks volumes for how good she is at being undercover. She maintained her secret for many years, but ultimately Kuroki decided to be more of a hero than a villain. Black Widow is a top-notch spy even if she occasionally fails, and Iron Man 2's assignment is definitely a joke compared to what she was expected to do in Demon Days.