Beretta/Relationships

Lamrys
Beretta was first viewing the protection of Lamrys as just it's job given by Rimuru, but they warmed up to her quickly. Guy coerced Beretta into "choosing" Lamrys as their primary master, with Rimuru as it's sub-master. As long as nothing happens to Lamrys, Beretta will only listen to Lamrys command.

Rimuru Tempest
Beretta possesses extremely high loyalty for Rimuru, to the point of fanatical devotion. Beretta has even declared the wish to serve Rimuru after serving Lamrys. When coerced by Guy to serve Lamrys as it's primary master, Beretta schemed to have Lamrys join hands with Rimuru, which makes protecting Lamrys considerably easier, lets them be close to Rimuru, and significantly reduces the chance of any situations to occur in the long term that could lead Lamrys and Rimuru into becoming hostile to each other, killing 3 birds with one stone.

Diablo
Beretta's progenitor. Diablo's held a grudge against Beretta for foiling his attempt at answering Rimuru's first Daemon summon ritual. He even attempted to wipe them out in secret, but Beretta tactfully diffused the situation by mentioning that their body was personally crafted by Rimuru himself, and that if he put a scratch on it he would earn his displeasure. Thus, Beretta successfully averted death.

Treyni
They both serve the same master and seem to get along together. Unlike Treyni though, Beretta is at least wary about excessively spoiling Lamrys.

Guy Crimson
Guy initially found Beretta to be suspicious as their loyalty was to two masters -- Rimuru and Lamrys. He forced Beretta to serve only Lamrys, and to his surprise Beretta agreed. Intrigued by this behavior, Guy tried to discern it's lineage and confirmed they're a descendant of Diablo.