I'm 90% sure yall are overthinking this and Marie's special someone is Stella.
I was waiting to see how long it will take before someone else speaks up. Really, you'd think the first possibility to come to a reader's mind was to assume that just like Mel has fallen in love with her master, her fellow beastkin Marie is probably in love with her own.
That was the first thing that came to mind. But other readers are right, too--while we, as always, get lots of lovey-dovey/jealous/overprotective looks between Master and Mel, the only person we actually see Marie beaming at is Mel.
Marie seems to be much more on a friendly teasing level with Stella, which certainly could be part of a more intimate "liking," but maybe not. And Marie ("Mary," in the earlier chapter's translation) was Mel's "very special friend" back in the beastkin training school.
Not that it says much about the possible depth of the Stella-Marie relationship, but I did also notice that there was another servant helping the girls dress--Marie is clearly in the role of "companion/helper" rather than servant or ladies maid.
And also notice where Mel puts her hands when she scoots away from Master--she's "creating distance," but that "tightness" that comes with her "special liking" may not be entirely in her chest.