Scarlet Widow

Scarlet Widow is the femslash ship between Natasha Romanoff and Wanda Maximoff from the Avengers fandom.

Avengers: Age of Ultron
Wanda took Natasha's jacket (revealed in a deleted scene). They fought alongside each other in the Battle of Sokovia. Natasha and Steve later began training the new Avenger recruits, including Wanda.

Captain America: Civil War
Natasha continued to train Wanda. The two took opposing sides over the Sokovia Accords and fought against each other at the Leipzig-Halle Airport.

Avengers: Infinity War
Wanda, Natasha, Steve, and Sam went on the run together, with Wanda occasionally leaving to meet up with Vision. After Wanda and Vision were ambushed by Proxima Midnight and Corvus Glaive, Natasha, Sam, and Steve showed up to save them. During the battle against Thanos' forces in Wakanda, Wanda leaped in to save Natasha and Okoye from Thanos' war machines. Natasha later returned the favor, teaming up with Okoye to distract Proxima Midnight from Wanda.

Natasha survived Thanos' snap to kill half the universe, while Wanda did not.

Fanon
The ship isn't as common in the MCU fandom. This is due to their more limited shared screen time. Wanda was also introduced later in the MCU timeline so many fans already had preferred pairing for Natasha. The two also don't have an established comic relationship for fans to pull from, unlike other characters who don't interact as much on film.

On AO3 it is the fourth most written ship for Wanda and the twelfth most written for Natasha.

Fandom
FAN FICTION