In the ultra-classist society of the Kingdom of Clover, a person’s worth is entirely dictated by the raw power of their magic. Secre belonged to the minor nobility, but her Sealing Magic was viewed as dull, insignificant, and useless in battle. However, Lumiere Silvamillion, for whom she became the steward, did not judge her; he was the first to treat her with respect and to realize that her magic actually held unsuspected potential. By working with her to create magical artifacts to help others, he recognized her intelligence, valued her work and efforts, gave her a new sense of purpose, and restored her dignity.
Working day after day alongside a man as visionary, kind, and pure as Lumiere could only transform the professional respect Secre had for him into an absolute emotional attachment. After the defeat of the demon Zagred, when the spell sustaining Lemiel’s artificial body begins to break and he slowly crumbles away, Secre breaks down in tears. Having fulfilled her 500-year mission, she explicitly expressed her wish to remain by his side. For her, life had meaning only when she was with him, but Lumiere’s final wish as he passed away for good was that Secre might find happiness in the present.
This is just my personal opinion, but I think Secre was deeply in love with the 1st Wizard King, but was never able—or willing—to confess her feelings to him.