Mina first shows up in Chapter 3, right when the Colossal Titan breaches Trost District. The massive explosion of steam from its body sends a bunch of trainees flying off the Wall, but she manages to keep her footing and follows Eren when he rallies everyone to attack it. After the Colossal makes its second move, she gets placed in the middle guard unit alongside Eren and Armin.
The three of them started out doing standard training corps work, though they got shuffled into the front lines pretty quickly once it became clear how many titans were flooding the district. Things got hairy when a titan caught them off guard and lunged at the rooftop they were standing on—a nasty moment that showed just how unprepared they still were.
By the time Eren starts talking about how taking down titans could get them promotions, Mina's already decided she's in. She joins him in signing up for the Scout Regiment, clearly drawn to the same idea of getting ahead through direct combat.
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