Beelzebub IV

, full name, and abbreviated to , is the Demon Lord. Beelzebub is the son of the Great Demon Lord.

Appearance
Beelzebub is a baby with an abnormally wide build and green hair. Beel is almost always shown with his binky, save when he is being fed. Usually, he is seen nude (even in public), causing ordinary people to talk about him and his "parent" Oga. Partially due to Hilda's misdirection, the rest of the Oga's family believes that Beelzebub is Oga's biological son, using their similar eyes (usually frozen in either a blank stare or an intimidating glare)

Personality
Baby Be'el is very easily impressed, especially by strong people since he himself is still weak and loses to other babies as well as to cats. On the other hand he is not the smartest one out there and tries to force his way through the most simple problems. It is also shown that he is afraid of insects. He often displays a fondness for things that might frighten "normal" babies, such as violence, exposed muscle tissue (on an anatomy model), heavy metal music, and demonic toys.

At the start of the series, Baby Be'el used to be a crybaby who used to cry and shock Oga all the time, but as the series goes on he gradually becomes braver to a point where he is now rarely seen crying at all. This is due to Oga's continuous teachings about what it is to be a man and a Demon Lord.

Touhoushinki Arc
Beelzebub came to the human world inside of Alaindelon. He was practically drawn to Oga Tatsumi from the beginning and is usually seen clinging to his back. Oga in turn did and still does not want to keep him around since he only causes him problems with other strong people at his school.

Oga tries to give him to Kanzaki, but later Oga beats him up for the disrespectfulness to his comrade. Next day he faces with the serious problem from Beel. Beelzebub pees up like flood with his ability and Oga cries over his fortune again. They encounter the problem with an equipment from the demon world for two hours or so.

Beelzebub and Oga head out to a department store to get the Diapers, so the pee of Beezebub would be stopped. They encounter a gang of robbers named as Orz3. Beelzebub gets attached to one of the Robbers, but Oga, feeling uncomfortable over the behaviour of robber, kicks him out. The baby starts to pee over and starts flooding, but later Hilda takes them along to the ocean for rescuing Japan from flood.

Saint Ishiyama High Arc

Electrocution
When Baby Be'el is caused to cry for one of various reasons, for example Oga being in danger, it fries everything in a certain range depending on how loud he cries. Generally the energy used is higher than that of a 12.000 Volt stun baton Himekawa of the Touhoushinki uses against Oga in chapter 9. Initially, if Beelzebub was more than 15 meters away from Oga, the electric shock would reach a fatal level (for Oga). Due to their new-found trust, this distance has increased, but only by 8 centimeters. Recently, Be'el has shown the ability to control his lightning and shoot it with enough power to defeat a demon.

Pee
Once a year Baby Be'el relieves the blessed urine that contains holy demon power. It can't be stopped by simple means such as knotting or diapers and would be able to sink a whole town. This however was prevented by Oga and Hilda riding on Ak'Baba to the ocean.

Giant
In the Demon World, Beel was able to turn into a giant and defeat the monster of the land.

Hidden Potential
While training with Saotome Zenjuro, a radio was used that made a "shadow" of Be'el. This shadow supposedly embodies the potential of its subject, and, as it was able to easily strike Oga into a boulder and crush that boulder, Be'el is hinted to have a large amount of fighting potential. Later, when Be'el and Oga switch bodies, Oga is seen defeating numerous delinquent's using Be'el's body, implying that Be'el already has great fighting powers but doesn't know how to control his strength.

Super Milk Time
It is revealed by Oga that he is not the only one getting stronger and that Be'el also receives a large power boost when they use Super Milk Time and they also fused after using the "Super Milk Time". At 30 CC, he was able to block the axe of Basilisk, one of the Pillar Heads, and at 80 CC, he was able to shatter the axe. Super Milk Time is a time limited technique.

Ankokubutō
Ankokubutō is a technique that allows possession of the body. It is speeding up Be'el's maturation process, as he has his urination period 6 months to early. It also causes him to start teething early. This process might lead to complications later on.

Hilda
Beelzebub is close with his Maid servant, Hilda, as shown when she was stabbed by Hecatos, he cried and even though Oga told him not to cry because Hilda is still watching them, he was still on the verge of crying. He also refused to run away when Hilda told them to run away. Also at the first series, Hilda asked if she should come with with Oga to school, Oga refused. That caused Beel to cry and electrify Oga for not letting Hilda come with them.

Tatsumi Oga
Oga is Beelzebub's chosen adoptive father, selected both by Arandron and Beelzebub himself. Though they get off to a somewhat rocky start, even Hilda is surprised by the strong bond that rapidly develops between Oga and Beelzebub. When not crying or otherwise upset, Beelzebub actually listens to Oga and recalls his lessons on being a man clearly. Likewise, Oga picks up Beelzebub's speech patterns quickly and gets much better at taking care of him. Oga initially only wants to find a stronger parent for Beelzebub in order to get rid of him, but as they grow closer, this search is implied to be more for Beelzebub's own well-being than anything else: he needs a strong, qualified parent to draw out his full demon potential.

Aoi Kunieda
Beelzebub is very attracted to Kunieda. When Kunieda displays her strength, Beelzebub was happy. He also seems to be comfortable with Kunieda. When Kunieda was holding him during Oga's fight with Shinjo, he fell asleep in her arms, or when Beelzebub in Oga's body, he smelled demon aura from Kunieda and grasped her twice. Kunieda is also one of the few people capable to hold Beelzebub without him crying. One being Oga, and the other being Hilda. Since Beelzebub is also quite attached to Aoi, Hilda tends to be quite jealous of her.

Trivia

 * In Animax broadcasts, Baby Beel wears diapers over his genitals.