Tatsumi Oga (男鹿 辰巳, Oga Tatsumi) is the main male protagonist of the series Beelzebub and is a first-year student at Ishiyama High School.
He is the adoptive father of the Demons, Beelzebub IV, and his younger sister, Nico. Early on, he attempted to pass Beel off to others on numerous occasions since unknowingly forming a contract with him.
In the Japanese version, he is voiced by Katsuyuki Konishi and in the English version, he is voiced by Andrew M. Gray.
Oga is usually seen sporting a grotesque grin and displays demonic features when he is either angry or disturbed. He possesses brown hair and sharp black eyes, he is rather muscular with a widespread build, and his attire usually consists of a worn-out shirt, his brown leather jacket, and plain pants (usually jeans).
He has been shown on a few occasions to be wearing either sweatpants, and on one occasion, has even been shown wearing his pajamas (ironically very detailed with pinstripes going down the side). When Oga attempts to smile, he is usually seen with a demonic, evil grin. This speaks to his lack of social skills (due to his inhibition in a juvenile delinquent high school).
When fighting, Oga appears to have a different look. Aside from his rare comedic entrances (usually as a relief from an intense situation), he is usually very calm and actually quite aware. His eyes are alert and he seems in control of his emotions. Beel does not transfer this emotion, however, and is usually shown in a maintained and alert state.
When he uses Super Milk Time, he becomes one with Baby Beel. His hair is green and he is sucking the pacifier Baby Beel uses. He is the same height that he is in his original state, and wears the same clothes.
Initially, Oga was nothing more than a crude and uncaring delinquent whose only friend seemed to be Furuichi. While he wouldn't start fights, he would take deliberate pleasure in inflicting the most pain possible when facing his opponents, regularly resulting to cruel and unusual methods of punishment for his defeated enemies, showing a high amount of sadism. It is because of these qualities that he was initially chosen to become Beel's parent.
Alaindelon described the ideal parent of a Demon Lord as being "strong, tough, self-centered, and cares nothing for his fellow man". These traits sum up most of Oga's personality. As the series continues, it should be noted that the trials Oga has faced and the new friends he has gained have somewhat altered his personality. For example, when the Rokkisei called out Ishiyama to meet them on the rooftop, he initially came alone. But later on, when Himekawa, Kunieda, and Natsume arrived, Oga smiled, knowing that they have his back. Though he retains his love for his cruelty, he is now much calmer and easier to be around than he was at the start of the series.
Still, in his own twisted, roundabout way, he seems to care for the people that are close to him. When Furuichi was gone during the summer vacation and left Oga alone, he called Alaindelon to transport Furuichi back home just so he could hang out with him. He also beat up and embarrassed Miki back in middle school, though he did it to protect him from Kiriya Reiji. Since he knew that both Miki and Kiriya would move to Nara at the same time, Kiriya would've attacked Miki in Nara where Oga wouldn't be there to protect him. When Hilda and Beel went missing and his sister Misaki threw him out of the house, promising to not let him back in unless he brought them back, Oga acted as if he didn't care. Instead, he complained about the heat to Furuichi and decided that they should go down to the river. Furuichi realized that the river was where Oga had first met Beel, also realizing that Oga meant to look for them after all.
Oga doesn't seem to have any sense of appeal for or opposition against girls, unlike Furuichi, who is easily seduced and will not hesitate to pick up girls. However, he does also have a sense of chivalry, as said by Hilda, as Oga will not fight or harm a woman. This was proven true when he only dodges during his fight with Aoi, and more recently when fighting the female Pillars he held back when he realized he was about to hit a girl. The only times he actually hits a girl was during a sparring match with Aoi to determine if they were capable of learning Black Techs, and knocking out Yuchelle by destroying her weapon. It is also hinted that he disapproves of underage smoking and drinking.
As much as Oga hates it, he tends to listen to lectures very well. For example, when Oga was telling Teimō about Baby Beel's demon praying dance, Kunieda quickly takes him to the side and lectures him about being selective about who to talk to about Demons, and to only talk about them to those who are involved. After that, Oga never told anyone besides Furuichi, Kunieda, Ittōsai, and Saotome, save for other demons. Another example of Oga paying attention is when Saotome was lecturing him about using Super Milk Time for no more than 5 minutes. When he uses Super Milk Time later, he only ended up fighting for 5 minutes after using a huge blast that knocked him unconscious.
As the series progresses, Oga learns how to put his pride aside and knows when to follow orders. An example was when Saotome said that the Ishiyama students were going to need training with him. All of the delinquents didn't want to train with him because they thought he was an "old geezer", and that he was weaker than them and it would damage their pride. Oga was the only one who stepped up and agreed to train; instead of being arrogant, he listened to reason and sought to become stronger.