By harvesting the souls of the beasts they had just defeated, Kyle was testing how much Soul force it required to accelerate the Soul draining effect of Starbreaker.
In the end, he figured that he was unable to reach the limit of the acceleration process as of now.
Nonetheless, it was possible to give it a slight push with only traces of Soul force, which was already interesting enough.
His eyes flicked to Beatrice and Yasmine who were breathing heavily, which caused him to smile lightly.
Yet, he didn't even think of invigorating them.
That was because they wouldn't even consider holding back their strength anymore.
He didn't want the two women to start relying on his ability because he was trying to prevent doing the same with himself.
It was also the reason for him to be breathing hard with the sole difference that he didn't expect to be nurtured by the Nurture ability as it was the case for Yasmine and Beatrice.