"Allen, I don't care about Asturia or Freid. Just so long as you're safe." -
Millerna
Allen Schezar and Millerna Aston.
When she is first introduced, Millerna is madly in love with Allen. He does not outright return her affections, and moves on to trying to reject her advances after she becomes betrothed to Dryden Fassa. However, likely because he sees Marlene in her, he does not completely stave off any romantic gestures toward her, such as letting him kiss her on the rooftop of the castle, and kissing her before he leaves Palas.
Millerna runs after Allen to Freid after he is exiled from Asturia. When he is injured, because she is the only doctor around, she helps treat his wounds and cares for him while they are in Freid. However, while at her eldest sister Marlene's manor, she reads that Marlene and Allen were the parents of Chid Freid. Shocked that Allen fathered an illegitimate child, and having recently been introduced to her betrothed Dryden, she confronts Hitomi for advice, and upon Hitomi's confession that she likes Allen, claims that he's "too far out of her league."
Near the end of the series, Hitomi encourages Millerna not to give up on her love for Allen, but she goes through with the wedding after Hitomi gives her a false, but hopeful reading of her future. Eries encourages Allen to let Millerna go, because she's happy with Dryden, and he tells her he agrees and moves on to a short-lived relationship with Hitomi.
In their final scene together, likely precipitated by the affection and devotion Dryden shows to her and her process of growing up throughout the series, it is implied Millerna has moved on from Allen, and that the same is true of Allen himself.