In Escaped chasm, Zera has shown that he has the power to move to a another world than his world.  And in Dweller's empty path, he killed the Dark lord alone.

He's strong enough, but he wants to make Yoki's abilities his own.

In my opinion, Yoki's ability to calm the beast is actually the ability to control beastes.

This is a skill that Zera doesn't have, and Yoki herself doesn't perfect control it. so zera want's to train Yoki

how do you think?

It seems that Yoki is from another world too and maybe thats why Zera isnt able to make her abilities his own. He clearly has big plans and he need Yoki's power for them.