Trust?
lol Who cares about trust.
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Reason why I like Miri and see her as the best girl is because of her personality.
She, out of everyone in the MC's squad has the least vanilla (and I don't mean this as any offense, cause it's perfectly fine to like vanilla. Just a matter of personal tastes) personality out of them all.
Heck, even if she hadn't been forthcoming to MC about her spying, she'd still be my favorite (unless she literally stabs MC in the back in the most painful way lol).
That, along with the fact that she looks smoking hot (yes, other girls are really nice too, but for me Miri just takes the cake for the best LI design as well.