Faced with his sister's question, all Klein could do was reply with a regretful smile, "Sore muscles."
He originally believed that by consuming the Sequence potion, his constitution would be enhanced as a Beyonder, but the harsh reality told him that a Seer's stats points were all allocated to his spirituality, mind, intuition, and interpretation. It didn't aid him in adjusting to combat training quickly.
As for the original Klein, he had focused on his studies early on and had suffered from malnutrition. That led him to possess a below-average physical condition. The fact that he was having 'after effects' from working out was to be expected.
"Sore muscles? I remember you returning after dinner last night and you didn't do anything else… Does alcohol cause sore muscles?" Melissa inquired with an inquisitive look.
