When she first met Rimuru, she was doubtful of him and even contemplated killing him. To test his strength, she unleashed her Demon Lord's Haki at full power, but Rimuru didn't even flinch, which greatly impressed her. After Rimuru named her Carrera and granted her a physical body, her loyalty towards him became unwavering. In a display of her loyalty, when the Scorch Dragon Velgrynd offered them the opportunity to surrender and join the Empire's side, Carrera, along with the other Three Devil Girls, adamantly refused. Following the events of The Abyss Unleashed, Carrera made a solemn vow that she and her entire family would serve Rimuru for all eternity.
In the Daemon Realm, they were always competing and battling with each other. Now they are competing for Rimuru's attention. Like Ultima she thinks that Testarossa is a cunning fox and cannot let her guard down when around her.
Carrera is constantly at odds with Ultima. In the Daemon Realm, they were always competing and battling with each other. The only time they weren't at odds with each other was to stop Testarossa from getting ahead of them in their game for supremacy in the daemon realm. For now, other than competing for Rimuru's attention, they're bickering over the most trivial matters on a daily basis. Their daily fights even become an attraction and subjects for betting.
In the Daemon Realm, they were always competing and battling with each other. Now they are competing for Rimuru's attention. She's often beaten up by Diablo together with Ultima as a form of "mediation". In order to repay the favor to him for introducing her to Rimuru she follows his orders but still thinks he's annoying.
Their territories are close to each other. As such, Carrera's given him his fair share of headaches dealing with her antics. The reason why Carrera kept attacking his territory was to get him to fight her since Guy acknowledge his strength.
