"Sister? Are you here for brother?" Onyx had felt the very same thing as Midnight. What Walker had not understood initially was that his bonds with Onyx, Fleur, and Midnight were all intertwined with his soul to deeper depths than anyone could comprehend. Therefore, when Walker began focusing on the bond he had with Midnight, he had started to enhance the bond with Onyx as well. Not to mention, with Fleur.
Thai made Onyx feel the same determination and drive to run to Walker's side. To fuel himself to grow and stand taller. The only reason Onyx had remained where he was, was the fact that he had made the call to remain with the hatchlings. Onyx also did not dare to interrupt Walker's training. Walker had committed to training alone for a reason and if he ruined that, Onyx would be unable to cope with his feelings. He would have let Walker down.