Beretta/Chronology

Human-Monster Interaction Arc
They were summoned to inhabit a golem created by Rimuru to protect Lamrys and the Labyrinth. The length of the contract is a hundred years. The payment for the contract was Rimuru's magical energy, which was already received from the summoning. Afterward, they could keep the body or throw it away. They requested that after it has protected Lamrys for a hundred years, that they be allowed to serve under Rimuru. And if he only promised it this, it would never bother him with another payment.

The Octagram Soars Brightly Arc
Later Guy Crimson convinced them (with a bit of a threat included) that serving two masters at the same time is a bad idea and that it should rather choose one true master to serve. Beretta agreed to truly follow Lamrys from that point on, but ultimately Beretta had it's own idea on how to "implement" this. They led Lamrys into making the Jura Tempest Federation their new "headquarters." One thing led to another and in the end Lamrys recreated the Labyrinth space underneath the Tempest Collosseum, managed together by Lamrys and Rimuru, making Beretta's plans a success.