Well, Cid Kagenou does reveal himself at a crucial point in the manga. This reveal has significant implications for the plot and character development.
Their relationship can be developed through shared adventures. For example, if they go on a quest together to retrieve a powerful artifact. During this journey, they have to rely on each other's skills. Cid's combat and strategic skills combined with Rose's knowledge of the local area or certain magic can bring them closer. They start to respect each other more, which is the first step in developing their relationship.
I don't have a definite answer for you. The manga might be keeping us in suspense about who Cid Kagenou will end up with. It could be a surprise twist or a character that has been hinted at throughout the series.
One plot idea could be that Rose discovers Cid's true power in a moment of crisis. For example, when facing a powerful enemy that the Shadow Garden can't easily defeat, Cid reveals his real capabilities to save Rose and the others. Another idea is a love triangle where Rose has feelings for Cid, but Cid is so focused on his shadowy plans that he doesn't notice at first, while another character also vies for Rose's attention.
The Eminence in Shadow light novel is finished. It provided a conclusive ending for the characters and their adventures, leaving fans with a complete narrative.