Beelzebub is a demon of Judeo-Christian tradition who is a derivation of a Philistine god known as Ba'al Zebul. As the monotheistic Hebrews were critical of what they perceived as false idolatry, they began applying the name Ba'al Zebub to that god, where "Zebub" means "fly." And as "Ba'al" was a generic term within Semitic religion for the title of "Lord," Ba'al Zebub can then be literally translated as "Lord of the Flies," a name by which Beelzebub is still known today.
Later Christian writings included Beelzebub in the demonic pantheon, often giving him great importance in Hell. The English poet John Milton provided an influential portrayal of Beelzebub in his epic poem Paradise Lost, where he is Satan's second-in-command. Beelzebub's name is sometimes equated with Lucifer or Satan, but may also just as frequently be portrayed as a separate demon. He represents the sins of Pride, Greed, or Gluttony depending on the source.
Imagery of Beelzebub was influenced by a woodblock print of the demon in the 1863 printing of the French demonic compendium, the Dictionnaire Infernal. This print featured Beelzebub as a giant fly with skull and crossbones on his wings, symbolizing his association with death.
In Final Fantasy
Beelzebub appears in Final Fantasy II's final dungeon, Pandaemonium, guarding a chest that contains the Genji Armor. In the story he is implied to be working under Emperor Mateus, who himself is under the influence of an unseen Satan, along with fellow demon lord Astaroth.
Designed by Yoshitaka Amano, Final Fantasy II's Beelzebub is simply a large, demonic fly. This version of Beelzebub would appear in all subsequent remakes of the game, as well as Final Fantasy IV: The After Years.
In Megami Tensei
Beelzebub is seen as an underling of Lucifer in the Shin Megami Tensei series. He is often portrayed initially as his human-like form Baal Zebul, though he may also be considered a separate entity altogether. When he appears in the series, Beelzebub is almost always part of the Dark-Chaos Tyrant race. He is also frequently encountered in the series as an optional boss.
As the human-like character was designed first for the original Shin Megami Tensei, there is some design similitude between it and his fly form introduced in the sequel, since both were designed by series artist Kazuma Kaneko. The pair hold skeletal scepters, wear skull necklaces, and have red eyes with Zebul's bluish skin darkening into a deep purplish exoskeleton upon his demonic exultation. Additionally, the human form also wears a tiger pelt, while this pattern also appears on the fly's head.
Megami Tensei main series
Beelzebub appears as early as Megami Tensei as a member of the Vile race, in the form of a relatively small fly. In Megami Tensei II he was moved into the Tyrant race for the first time as a level 110 boss and adapted into a much more monstrous, insectoid design with a human face and fly's head emerging from its chest. Likewise, Beelzebub is also in the original Megami Tensei remake compilation Kyuuyaku Megami Tensei for the Super Famicom.
As noted above, Beelzebub's human form still seen in the series today marks its first appearance in Shin Megami Tensei, where he is the top seed of the Tyrant race. In Shin Megami Tensei II his true fly shape is encountered during the story, where he awaits in Lucifer's Kether Castle. After challenging Aleph and losing, Beelzebub mentions his body's temporal state and invites Aleph to resummon him through fusion in the Cathedral of Shadows.
Shin Megami Tensei: Nocturne
In Nocturne, Beelzebub is the boss of the 4th Kalpa of the Labyrinth of Amala. He is sent by his master, Lucifer, to test the will of the Demi-Fiend in battle. Beelzebub can only be accessed in a vortex of 4th Kalpa known as the "10 Meters of Hell," which will reject the Demi-Fiend except at certain phases of Kagutsuchi; Beelzebub's chamber is accessible at New Kagutsuchi.
Once reached and after first addressing the Demi-Fiend in his human form, Beelzebub will transform into his more iconic fly appearance and do battle. Beelzebub resists all damage but Fire and Almighty attacks and, after losing enough HP, will start casting his skill Death Flies, which, in addition to high Almighty damage, will instantly kill any member of the player's party that does not void Death skills. A specific music track simply titled "Beelzebub" plays during his boss fight and is an arrangement of the Chaos theme from Shin Megami Tensei.
Once beaten, the player can fuse Beelzebub in the Cathedral of Shadows. He must first be fused as his human form, which will then transform into his more powerful fly visage after reaching a high enough level.
Beelzebub is the final boss of the infamously rare Virtual Boy game Jack Bros., where in the English version his name was changed to "Belzoff." As one of the three Jacks-- Frost, Lantern, or Skelton--the player must defeat the human form before confronting the fly in a decisive showdown to save the land of the fairies from his tyranny.
Devil Summoner series
Beelzebub appears in Devil Summoner: Soul Hackers as the boss in the optional end-game area the Temporal Distortion, along with the likes of King Frost, Mara, Lucifuge, and Lilith. The last of the bosses in that sequence, Beelzebub uses a high-powered Mazionga attack and has high resistances. Upon his defeat, New Game+ will unlock for the player.
In Devil Summoner: Raidou Kuzunoha vs. The Soulless Army and vs. King Abaddon, Beelzebub is a member of the Pagan demon clan. In The Soulless Army, he is also a high-level optional boss.
In Revelations: Persona, Beelzebub is a level 77 persona of the Devil arcana, the same arcana he is a part of throughout the Persona series, though his name was mistranslated, perhaps intentionally, as "Beezlebum." This would be later corrected in the game's PSP remake. He also appears in both Persona 2: Innocent Sin and Persona 2: Eternal Punishment.
In Persona 3, FES, and Portable, Beelzebub is the reward for achieving max S. Link rank with the seedy TV salesman Tanaka. Beelzebub is a level 81 persona of the Devil arcana and must be fused in a hexagon spread using Incubus, Succubus, Pazuzu, Lilith, Abaddon, and Baal Zebul.
Similar to his appearance in Persona 3, Beelzebub is available as a persona in Persona 4 after reaching a max-ranked S. Link with Sayoko Uehara of the Devil arcana. Again, he can be fused only in a hexagon spread using the personas Belial, Belphegor, Baal Zebul, Seth, Mot, and Pazuzu.
Beelzebub also appears in episode 17 of Persona 4: The Animation. Backed into a corner by Naoto's Shadow, the main character fuses Beelzebub in real-time, who then uses Megidolaon to blow Naoto's Secret Base into oblivion.
Devil Survivor series
Beelzebub, as Belzaboul, is a Bel boss faced by the player on the 7th and final day of Devil Survivor and Overclocked. He mentions he has no intention on competing for the throne of Bel as he was merely sent by Lucifer to ensure that Belberith take the seat. In battle, Belzaboul can inflict party members with the fly status, which induces fly spawn to grow within units that do large damage when they erupt and cause extra fly spawn to appear on the battlefield.
Beelzebub reappears in Devil Survivor 2 as an optional boss battle on Friday within the game, though he can only be faced on a second playthrough. He seems to view the battle as sport, and summons a myriad of high-level enemies to aid him. After each grouping of enemies is defeated, his elemental affinity will cycle through a weakness, from Physical to Fire to Ice and so on. After being defeated he becomes available for fusion as a member of the Tyrant race.
Other Megami Tensei games
Additionally, Beelzebub is a member of the Tyrant race in Majin Tensei II: Spiral Nemesis, an optional boss again in Digital Devil Saga, and makes numerous appearances in the Devil Children subseries.
In Castlevania: Symphony of the Night, Beelzebub appears as an optional boss in the Inverted Alchemy Lab. This iteration of the Lord of the Flies is in the form of a giant humanoid bound together by chains. Because he is immobile, Beelzebub will instead summon swarms of grotesque, giant flies to attack Alucard (or Richter or Maria). As he takes damage, Beelzebub's partitioned limbs will unbind and fall off until just his upper torso with exposed innards remains.
This same version of Beelzebub was reused in the online multiplayer game Castlevania: Harmony of Despair. He appears as the boss of DLC Chapter 9 and attacks players with fast and complex fly swarms.