Bertolt and Mikasa were both from the 104th trainees.
Reiner told Eren that he and Bertolt wanted to talk to him, and Eren stayed behind as the group went to rest. However, Mikasa stayed where she was, listening closely to whatever Reiner had to say. When Reiner revealed that he was the Armored Titan and Bertolt the Colossal Titan, Mikasa's stance was still steadfast until Reiner unwrapped the cloth from his arm and revealed his wound healing at a fast pace. Bertolt screamed if they were doing it now, and Reiner yelled back they were going to settle this. Mikasa immediately jumped in and slashed Reiner's arm and went for Bertolt, cutting his neck slightly. As he fell onto the ground Mikasa stood over him and was about to finish him until Reiner came and pushed her away and off the wall. Mikasa used her maneuver gear to save herself from falling to her death, and she glared up at the two. It later seems that Mikasa didn't kill them because she saw them as comrades and couldn't believe it.
When Eren was captured, and Bertolt was inside Reiner's hand with Eren tied onto his back, Mikasa appeared and gave Bertolt a death stare. Bertolt stared back terrified. The others gathered around Reiner's hand, where Bertolt and Eren were, and they asked Bertolt why he and Reiner were doing this to them. Bertolt yelled that this was not what they both wanted to do. He said they had no choice and begged somebody to save them. After a short quiet moment, Mikasa demanded to give Eren back. Bertolt said that he couldn't and that he had no choice but to bring Eren with them to their hometown.
Another slight interaction between Mikasa and Bertolt was when Armin tried talking to Bertolt, asking him if this was what he really wanted. Bertolt replied that he had no choice but to do this, calling them devils. Bertolt then told Armin that he was going to die and slowly walked towards Armin. Mikasa then appeared from behind and went for his neck, but Bertolt blocked her blow by blocking the sword with his own. As they fought, Mikasa cut his ear, and Bertolt did a roundhouse kick. He successfully managed to escape as Mikasa returned.
This controversial ship still managed to get a couple of shippers and fan arts, despite the few interactions Bertolt and Mikasa had. Some find this ship unusual since Bertolt and Mikasa showed almost no sign of interest to one another. Others says that Mikasa clearly despises Bertolt, and therefore this ship makes no sense to the fans. Because of their limited interactions, this ship is one the more uncommon rarepairs in the fandom.