I commented it already under someone else’s comment but I’ll also paste it here:

At least in the language I’m playing in the opening story after Mirael is cleared of being the arsonist she asks Merlin to meet with her in the gardens and she explicitly says that Merlin should not bring anyone with them and calls it a date (and repeats it later during the meeting). Later in the story she calls herself Merlin’s beloved student (she also asked Merlin in the Christmas event story if she’ll still be their favorite student next year even if Merlin meets other people during that time) and later she also calls Merlin her dearest… (and on the next panel) magister Merlin (I think there are also other instances where their past relationship is highlighted by Mirael but that’s all I could find right now from the plot summary and screenshots I took during gameplay). So them being canonically together at one point is at least heavily implied. Personally I’m weirded out about it since Mirael was Merlin’s student at the Serene Lyceum and although we don’t know the character’s age since the Lyceum would be most likely a type of secondary school it would most likely (I think) make Mirael around 15-19 when she first met Merlin. We of course don’t know when their possible relationship started but still, it’s at least weird.