While everyone was training, Ren was sitting cross legged on the grass. He was also training by looking into his soul. At his current state gaining more mana wasn't possible since he already reached the limit of what his immature human body can handle. To strengthen himself with mana Ren had no choice but to wait for his body to mature. Only when his body grow up can he store more mana, then refine and strengthen it. Right now he was finding ways to make himself stronger without waiting for his body to grow.
So the only thing that could make him more powerful at the moment were techniques. Currently the only techniques from his demon lord days that his body could handle were limited. So he wanted to learn more moves from those powerful people in his visions. The majority of those guys were human, surely they had techniques that would suit his current human body.
