He appears much like a typical angel, dozens of feet tall, with white feathered wings and a third eye on his forehead. Mundus usually resides inside of a large marble statue, which is how both Dante and Trish see him for the first time. Mundus also has a form where he appears as three glowing orbs (eyes) arcing with energy. He is seen this way while communicating with Griffon, Trish, and before battling Vergil.
Mundus is depicted as the epitome of evil. He has no loyalty or compassion for his minions, ruthlessly killing one of his own generals for being unable to defeat Dante and pleaded for his master's aide, or even use Trish as a bargaining chip over Dante to ensure an easy victory, which could be seen as cowardly. Like most other demons in the series, Mundus has a low opinion on humans, even going so far as to wonder if Dante's human heritage had weakened his father's blood.
Mundus is also very driven by his goals, setting servant after servant to destroy Dante before finally taking the fight to the devil hunter himself. And even after losing again, he still forced a smaller Hellgate open in a bid for vengeance on Dante.
Two millennia ago, Mundus planned to lead an assault on the human world but the powerful demon knight Sparda awoke to justice, leading him to single-handedly defeat both Mundus and his armies, and seal the main gate within the Underworld. In response, Mundus planned revenge against Sparda's family. His demons attacked and killed Eva who was Sparda's wife, and Dante and Vergil's mother.
In the post-credits ending, as Sparda's son Vergil found himself trapped in the demon world, he comes upon Mundus, who glares down on him silently with his triple eyed avatar. Vergil challenges him in battle, though this battle is never seen. In his weakened state, Vergil is defeated, and transformed into Nelo Angelo, leading into the events of Devil May Cry.
Mundus attempts to escape into the human world by opening a gate on Mallet Island. At the same time, he uses his creation Trish to manipulate Sparda's other son, Dante, into coming to the island so that he can be killed. For much of the game, Mundus remains dormant within the statue of his younger form in the main hall of the castle, with Dante writing the statue off as nothing more than a depiction of the inhabitants god.
He first appears after the third battle with Griffon, ruthlessly executing his fallen general after he pleads to Mundus to be healed and strengthened to defeat Dante. Mundus is not seen again until Nelo Angelo's ultimate defeat, commenting to Trish that Vergil had fallen and saying she knew what she had to do. While Trish does ultimately lure Dante into a trap to battle Nightmare, this fails and Mundus appears, threatening Trish with the consequences of her failure.
When Dante finally arrives in Mundus' lair, a chapel like area, and sees the Devil Emperor sitting on his throne, musing on the irony of again facing a Sparda, with Dante retorting the irony of Mundus' defeat at his hands. Mundus however has one last card to play: The captured Trish. Playing on Dante's feelings for Trish, Mundus ruthlessly impales Dante with three beams of red energy, taunting Dante's weakness. Mundus prepares to deal the final blow when Trish intervenes, seemingly sacrifcing herself for Dante.
Annoyed with his creation's 'behavior' Mundus again attempts to destroy Dante, but the young devil hunter deflects the blast and demands Mundus show himself. Deciding to honor Dante's words, Mundus sheds his marble like form, revealing his aged, angelic body and then transports himself and Dante to his pocket dimension. There, he comments on how he now sees the light of Sparda's eyes within Dante's own. When Dante demands to know why Mundus used his mother's face, Mundus calls her a 'worthless being' and suggests to Dante he could make as many as he wanted. Dante demands silence and this sparks the final battle between the two, with Mundus flying off into the darkness and Dante assuming his father's form via Devil Trigger.
Dante succeeds in injuring Mundus but himself is knocked down into the pit of lava below, Mundus following. Mundus attempts to overwhelm Dante with his raw power but Dante overcomes him after a lengthy battle, and Mundus painfully takes to the skies, screaming as his wings fall away and his pocket dimension fades away, seemingly taking Mundus with it.
However, the prince of darkness returns as Mallet Island begins to collapse, literally tearing open a much smaller Hell Gate in the sewers, directly in front of Dante. The grieviously wounded Mundus vowed Dante would not leave, and Dante decided to stand his ground, swearing the island would become Mundus own burial ground. Though his powers had been greatly weakened, and Dante was able to chip away at his body, exposing his hideous true form, Mundus only mocked Dante's efforts, calling his powers weak in the Human World.
But just then, Trish returned and gave Dante her powers, supercharging the devil hunter who blasted Mundus with his overpowered handguns, sending the Prince of Darkness screaming back into the Underworld. As he was forced to return to the demon world, Mundus vowed to Dante he would return and rule the human world.
In Ninja Theory's reimagining game, the alternate Mundus is a Demon King of immense power who is the ruler of all demons and most humans. He watches over Limbo City (through the CCTV system connected to a wall of video monitors in his large office) from a seat of power in both the demonic realm, and the human world, where he operates under the guise of "Kyle Ryder", a banker who uses Raptor News Network's propaganda to portray himself as a philanthropist. Despite the good public face he shares, he is quite ruthless, operating his business more like a loanshark than a banker, never giving extensions, and reinforcing repercussions for late customers.
- Mundus' name comes from the Mundus Cereris, which is a pit in Roman mythology that contains an entrance to the underworld Hades, which is ruled by Pluto, the Roman god of the underworld; Mundus is identified as Pluto twice within Devil May Cry.
- Mundus' three eyes are likely a reference to Satan, who is portrayed with three faces in Dante Alighieri's Divine Comedy. Satan is also considered the "Prince of This World" and the "Prince of Darkness", both of which are titles that Mundus shares with him. Mundus's pseudo-angelic appearance also coincides with Satan's traditional role as the fallen angel Lucifer within the Christian faith.
- In Marvel vs. Capcom 3, Mundus is mentioned in Dormammu's win quote against Dante ("You should stick to fighting Mundus!") and in Ultimate Marvel vs. Capcom 3, he is mentioned in Ghost Rider's pre-battle quote against Trish ("I eat demons like Mundus for breakfast.") and Phoenix/Jean Gray's win quote against Vergil ("Perhaps if I'm not too busy, I'll take care of Mundus, too.").