There are many notable characters in That Time I Got Reincarnated As A Slime, but none levy the most suspicion quite like Diablo, Rimuru the Slime's second secretary. Though he has proved to be loyal to Rimuru every step along their journey together, his predisposition as a Demon lends itself to him being thought of as untrustworthy. As Rimuru usually surrounds himself with only the most trusting and genuine of beings, it seemed surprising that Diablo was included among his closest allies, but Diablo is far from the typical demon. Diablo is one of Rimuru's greatest allies and there is no doubt about his loyalty.
Diablo is also a relatively new name for him, given to him by Rimuru. Before that moment, this being was known as Noir - one of the seven Primordial Demons. The power he wielded is immense, so much so that he actually stopped pursuing further power. He thought his fights were won much too easily.
Noir actually knew of Rimuru before being summoned by him and was very much looking forward to their meeting. Noir had come into contact with Shizue Izawa and was astonished by the power of her Anti-Magic Mask. When Rimuru was seen to later have that very mask, though now broken, Noir assumed that Rimuru was ultimately connected to the power of the mask. This was the initial draw for Noir as he believed Rimuru to be the "truth of the world" and felt destined to meet him.
Perhaps destiny or fate was a part of it, though Rimuru happened upon Noir on a seemingly chance encounter. After transforming into a Demon Lord in order to resurrect his fallen comrades after the invasion of Falmuth soldiers, Rimuru was extremely weak and losing consciousness. One last enemy prevailed, and with the last of his strength, Rimuru summoned a Primordial Demon. Noir, along with two other Demons, was summoned to his side, though Noir is the only one to finish the final enemy for Rimuru.
Now while at first glance it may seem shocking for Diablo/Noir to easily become subservient to Rimuru, when looking at their distinct personalities it actually makes a lot of sense. Diablo already had respect for Rimuru because of his belief to be connected to the power of the mask. That said, Rimuru commands the respect of his own accord. He's logical, rational, and tries to find the best solution for all involved. He's attentive, appreciative, and understands that battle is necessary at times. Diablo values power and Rimuru commands a lot of it, so it's understandable why Diablo so easily fell into his service.
Diablo had also always behaved strangely for a Primordial Demon as well. He's shockingly whimsical but can switch to a calculating, malicious Demon in the blink of an eye. He followed power, and after spending years feeling like he had achieved all that he could, seeing Rimuru's raw power, both in terms of battle and in political strategy, made Diablo fall right into line. Though he treated Rimuru as a superior, Rimuru considered him an ally, due to his Demon status and power level. Diablo and Rimuru may make surprising allies, but with Diablo at his side, Rimuru can be nigh unstoppable.
