Only his family (including tails bc well he their family doctor) knew Silhouette was mortal.
The family at first decided not to tell Maroon till they though he was old enough to handle the truth since they all thought Silhouette was immortal and all thought at least Maroon and Silho would have eachother but later test would confirm that thats not the case.
Silhouette came to that conclusion herself quite young and her parents did confirm it and asked her if she like to let Maroon know. Thinking if she could handle it maybe Maroon could too.
She and her family knew how soft he can be and he was so excited to live forever with Silhouette and his father. The thought that she may not live forever would crush him. She said no and so they left it up to her to tell him when she feels conformable with it.
As much as she may act cold and aloof at times, she dose care for her brother so she never could find a good time to tell him.
Maroon would later find out on his own which obviously broke his heart not just the news of Silho mortality but the fact his whole family didn't trust him with such important information all because of his emotions.
Obviously he runs off Shadow would be the one to find him first.
