Dietfried is stated to have a flirtatious air about him. Although he has a self-centered and bossy personality, he is social and charismatic, having some sort of quality that draws others to him. He is confident, never claiming to be afraid of anything and always being surrounded and respected by many people, never bashful of it. Since Gilbert could not love anyone due to being too stern, his elder brother had everything he lacked, to the point of making him infinitely envious as a fellow person. Dietfried is also easy-going, preferring to be playful and talk about fun matters rather than being confrontational. But he can also be very frank, and although he tries to come off as lighthearted, his words can be rude at times. Despite that, he can be compassionate; he loves his family deeply and values his underling's lives. He is also hardworking and influential, being the first Bougainvillea to join the navy. Not only had he joined it, but also steadily carved a career path for himself in it. It was all due to his confidence in scoring achievements with his own efforts and talents, even without making use of his parents’ glory.

He appears to have good leadership skills and responsibility, in which he can handle everything quite nicely. Although he possesses a sense of right and wrong and, he can also be cruel and merciless which is demonstrated by his actions towards Violet. He was aggressive towards her and saw her nothing more than a weapon, which is the reason he took her home. He had no problem with ordering Violet to murder people like a weapon and appeared to find enjoyment within this, though this stems from his hatred towards Violet for killing his men. Towards the end of the series however, Dietfried appears much calmer and less cold towards Violet.

Dietfried knows he has been doing horrible things, much worse than Violet had done for a living. But he doesn't care, he knows it will all be over when he dies either way. He has had his own have laments and hardships, and there were also times when he thought dying would be much better. At those moments, he even considered doing so. He feels really guilty for letting his companions die and blames himself for not being able to protect them. He also continues to visit the families of his companions that Violet had killed to give them money. He thinks that rather than dying, it’s harder to stay alive. Despite that, he is confident that he will live on, and expects Violet to do the same despite her mistakes.