Mira is often paired with Zoey, but what her character actually does is highlight Rumi's personality traits --and that makes Mira interesting.
Huntrix is organized in the classic three-man-band style: Rumi as leader, Mira as lancer, and Zoey as the heart. Lancers are active foils to the leader, their abilities compliment the leader's and their personality runs opposite to the leader. I.e. Bucky's brooding paired with Captain America's ray of sunshine, Glimmer's sense of whimsy clashing against Adora's practically.
So if Mira's default disposition is to play straight man to curb Rumi's enthusiasm (the Bert to Rumi's Ernie, if you will) then Mira's frustration throughout the movie makes much more sense. Why is Rumi running off and brooding? Why is Rumi grumpy and pushing for more work? Why is Rumi staying up late writing angsty music lyrics?
Mira is the black cat to Rumi's golden retriever, and the moment that stops Mira is concerned and frustrated.
So that's why Mira checks in with Rumi instead of Zoey despite Zoey's role as the heart. Mira feels the default literary trope is no longer working and she wants her job back.
But, ironically, by reaching out when Rumi is isolating herself, Mira still fulfills her role as a narrative foil and therefore maintains her position as lancer throughout the movie.
