It's definitely Reigen Arataka because he's really the realest mentor I've ever seen in anime, and despite the fact that he's a great fraud, I really admire his wide variety of skills that totally make up for it ಡ⁠ ͜⁠ ⁠ʖ⁠ ⁠ಡ. His influence on Mob's life principles is indeed admirable. He may be working in a shady business (most likely deceiving people), but despite that, he was still able to instill in Mob that he shouldn't use his powers for his own advantage. He's really a relatable character because he's not the usual almost perfect and powerful mentor—truth be told, he's totally a flawed one, but that didn't stop him from influencing and instilling good in others, especially to his very own loyal disciple, Mob.