By the time she was gone, Aito had already absorbed all four mana cores. He could feel his mana reserve filled to the brim. Before, it had only required two mana cores to fill his reserve. But now, after absorbing the level 2 soul core, fifty human cores, and some others, it was able to hold double that quantity.
However, for the purpose of leveling up, he absorbed one more mana core. The amount of energy circulating inside him pushed his soul core past its limit, forcing it to expand temporarily.
For what he had planned, Aito had no choice but to level up. It would enlarge his soul core, allowing him to use mana and his soul power for a longer duration. Moreover, it would instantly heal all his injuries and restore his stamina.
Just like the siblings during the fight against the sect members and him when he had acquired a class.