It's hard to say. Sometimes the manga keeps such revelations hidden until the right moment to create suspense. So, we'll just have to wait and see if Rose discovers Cid's secret identity.
The shadow in Qing Yu Nian was someone Fei Jie knew. The Shadow's true identity was Great Grandmaster Sigu Jian's younger brother and the son of the former Dongyi City Lord. Fei Jie had treated Sigu Jian in the movie, so he was very familiar with the people around Sigu Jian. The Shadow concealed his identity and became Chen Pingping's chief guard. He was also known as the world's number one assassin. Fei Jie wasn't surprised when he took off the shadow's mask, because he had known the shadow for a long time. The real identity of the shadow was Fei Jie, who was the behind-the-scenes worker in the drama.
The shadow in Qing Yu Nian was someone Fei Jie knew. The Shadow's true identity was Great Grandmaster Sigu Jian's younger brother. After Fei Jie removed the Shadow's mask in the play, he was surprised to discover that the Shadow's true identity was the Great Grandmaster Sigu Jian's younger brother. Thus, the Shadow was Great Grandmaster Sigu Jian's younger brother.
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.
In " Celebrating Years," the Shadow's true identity was Great Grandmaster Sigu Jian's younger brother. Elder Fei knew the Shadow because of his brother Sigu Jian. In the past, Elder Fei had treated Sigu Jian, who was not yet a Great Grandmaster, so he naturally knew the people around Sigu Jian. Therefore, Elder Fei was not surprised when he took off the shadow's mask.
In " Celebrating Years," the Shadow's true identity was Great Grandmaster Sigu Jian's younger brother. Elder Fei knew the Shadow because of his brother Sigu Jian. In the past, Elder Fei had treated Sigu Jian, who was not yet a Great Grandmaster, so he naturally knew the people around Sigu Jian. Therefore, Elder Fei was not surprised when he took off the shadow's mask.
In " Celebrating Years," the Shadow's true identity was Great Grandmaster Sigu Jian's younger brother. Elder Fei knew the Shadow because of his brother Sigu Jian. In the past, Elder Fei had treated Sigu Jian, who was not yet a Great Grandmaster, so he naturally knew the people around Sigu Jian. Therefore, Elder Fei wasn't surprised when he took off the shadow's mask.
In " Celebrating Years," the Shadow's true identity was Great Grandmaster Sigu Jian's younger brother. Elder Fei knew the Shadow because of his brother Sigu Jian. In the past, Elder Fei had treated Sigu Jian, who was not yet a Great Grandmaster, so he naturally knew the people around Sigu Jian. Therefore, Elder Fei was not surprised when he took off the shadow's mask.
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.