I think anyone can be in that tank, it's a basic job to try to cover a flag tank anyway.
By the way, Erika maybe in the tank Miho rescued but she didn't feel thankful for it.
My theory is:
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Erika back story
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Erika has been admiring Maho as her idol for a long time and tried her best to get on Maho's side as a vice-commander but there was one problem: Miho
Miho's skill as Nishizumi family member is too high for Erika to overcome no matter how hard she tried. Her dream to be a vice-commander was completely sealed off. Erika has been in jelious of Miho since then.
One day in the match with Pravda, her tank (She was the commander of that tank) that was covering Flag tank got hit and fell into the water and Miho had to help her which cost the victory of Black Forest.
Erika locked herself in her room, crying. While everyone seems to blame Miho for the lose, Erika realized that if her tank didn't fall in the water in the first place and made Miho (the one whom she hate the most) to come to help her, Black Forest won't lose.....While the tear was flowing from her eyes, an evil thought suddenly appeared in her head.
(Wait, It was Miho fault that abandon the tank not mine!! )
(Miho’s fame as Nishizumi is now ruined with the defeated of Black Forest; I can use this chance to prove that I’m better than her!)
Next few weeks, Miho quitted from Black Forest and went to Oarai, Erika’s future to become the Vice commander is now guaranteed.
"Maho-san, finally, I can be at your side ^^"
HAPPY ENDING