Grucius/Chronology

Human-Monster Interaction Arc
After Demon Lord Carrion made a non-aggression pact with Tempest, Grucius was sent to Rimuru City as part of Eurazania's delegation, taking the role of a carriage driver. As part of Albis' scheme to show Tempest's power to Eurazania's own delegation, Suphia picked a fight by insulting human, leading her to the situation where she is supposed to battle Youm but when they were about to fight, Shion ended up interfering due to not being able to tolerate the constant abusing insults she threw at Rimuru as provocation, causing her to fight against Suphia instead. Since Albis had to play the role of the calm and civilized negotiator, she involved Grucius so that he could take on the already battle-ready Youm, and both of the battles ended up in a stalemate after which Albis revealed her plan, which stopped the fighting after Rimuru successfully resolved the issue of Shion's Ogre Cannon being unable to be cancelled while it's already fully charged. Due to having bonded through the battle, Grucius and Youm became friends.

Afterwards, Grucius ended up being a long-term emissary who was supposed to be stationed in Tempest until further notice while the two Beastketeers returned back to Eurazania and Youm went back to his PR-campaign for Tempest as the Champion. While stationed in Tempest, Grucius ended up working as part of Tempest's Security Force for the duration of his stay.

Demon Lord Awakening Arc
When Youm returned to Tempest he brought Mjurran along with him. When Youm introduced her as his strongest officer Grucius didn't believe him and challenged her to a sparring battle. Grucius lost and due to a bet they made Grucius became Youms subordinate though Grucius notes that he'd still take Carrion's side in case of a disagreement.