Although he couldn't understand why a boy his age wouldn't want to be as flashy as possible, showing off all his capabilities to gain respect and fame, this made him realize that the boy was no fool.
There's a difference between being very strong and becoming an uncontrollable monster with a target on your back. Showing only what one needs to, and not all they can, ensures they don't make unnecessary enemies and can easily deal with any that do appear, because others would underestimate his true capabilities.
This was why Liang insisted that the squad leaders of the Reapers Guild be present when Kai was planning his little revenge after Alister had been stripped of his equipment, ensuring he couldn't summon any monsters.
Because Liang was certain that just taking away his equipment wouldn't be enough to stop him.
