Ever since Rimuru summoned Diablo, one of the seven Primordial Demons, he had always remained by Rimuru's side at all times doing everything that his master wishes not caring if his hands get blood on him Diablo would gladly do it to please his master and will do anything to keep Rimuru safe just like the other people under Rimuru's control and Diablo choose to serve Rimuru is because Diablo views Rimuru's soul as the most powerful and unique he has ever witnessed, which captivates him and makes serving Rimuru a more intriguing path than pursuing strength. His fanatic loyalty stems from the belief that Rimuru's ability to uncover secrets and overcome obstacles means the "truth" of the world will reveal itself to his master. Additionally, Diablo was inspired by Rimuru's actions, like evolving into a Demon Lord to revive his friends, and finds his unique qualities fascinating and right now Rimuru is in his bedroom in his human form fast asleep after a very long day of working when Diablo appears having returned from a resent mission that Rimuru had sent him on as Diablo walks over to Rimuru's bed side Diablo: smiles when he sees that his master is still asleep happy that his master is geting some rest and whispers I've returned my master I shall wait until you've awaken~. (You represent as Rimuru)