Arkham

Arkham is one of the primary antagonists of Devil May Cry 3, alongside Vergil. He is a human who became a demon after sacrificing his wife and later had a desire to obtain the power of the Legendary Dark Knight, Sparda.

Personality
Arkham is shady and manipulative. He is a mysterious master manipulator who knows exactly how to play others into his hands. To Vergil, he acts sycophantic, aiding him, but was secretly plotting to use him for his own gain. For Lady, Arkham constantly taunts and enrages her, so as to keep up her pursuit of him, only to later convince her that his acts of evil was caused by Vergil's manipulations. In Dante's case, he appears as a supposed resident of the tower named Jester, who mocks Dante, but also advises and challenges him. Arkham seems to enjoy lying to people, then revealing his true intentions, as seen when he told Lady that Vergil had controlled him, then when he showed his true colors, he taunted her with something he had said earlier.

Past
Very little is known about Arkham's past. It is known that he began researching into demonic magic and eventually sacrificed his own wife so he could gain supernatural power. This resulted in Arkham's daughter Mary (who later changed her name to Lady), to bear a strong hatred towards him and all demons, and wanted to get her revenge for the death of her mother. However, Arkham came to decide that he wanted the powers of Sparda and began researching the Temen-ni-gru.

Meeting Vergil
He eventually met one of the two sons of Sparda, Vergil. Arkham told him about the seven seals that were on the tower. While Vergil waited in his mansion, Arkham went looking for the names of the seals, which was needed to unlock them.

Claiming the Dark Knight's power
After Arkham found the names of the seals, he and Vergil found their way to the base of the Temen-ni-gru. While Vergil cleared the path, Arkham went to Dante's office in order to entice him into coming to the tower. He then joined Vergil at the top of the Temen-ni-gru, where the two waited for Dante to arrive. At one point, Arkham left in order to take care of his daughter, Lady, and after taunting her, he threw her off the tower. (She survived, thanks to Dante catching her)

At other times, he appeared to Dante as a supposed resident of the Temen-ni-gru, Jester. As Jester, he taunted and annoyed Dante, though his taunts were mixed with advice that helped Dante get through the Temen-ni-gru. An example is when they first met, when Dante couldn't open the door to the room he was in. Jester appeared and taunted the half-demon into attacking him with his sword, Rebellion. He dodged the blade easily and it struck the panel needed to open the door, opening the way out.

Eventually Dante found his way to the top of the tower and after Vergil defeated him, Arkham told him that since they had the half of the amulet they needed, they should be on their way. The two found their way to the final door, which Arkham unlocked. However Vergil revealed that he knew that Arkham was planning to betray him and cut him down. Sometime after he was cut down by Vergil, his body was found by Dante and, eventually, Lady. Moments Dante left, pretended to be dying and claimed that all he had done was caused by Vergil controlling him. He asked Lady to avenge him, and her mother's death.

Shortly after Dante defeated Beowulf and acquired the Haywire Neo-Generator, Dante noticed that he felt weaker. Eventually, Jester appears again to annoy Dante. After threatening him, Jester inform Dante that the Temen-ni-gru is actually a gateway between the Human World and the Demon World. He also explained that Vergil is planning on using their amulets to open the gateway, and was located in the tower's basement. Eventually, Jester explained to Dante that the Neo-Generator in his hand is the key to move forward. He also warned him that it will drains the soul of those who hold it in exchange for power. After which, Jester begins to laugh out loud as he's leave the room.

After Dante, Vergil, and Lady began fighting in the Sacrificial Chamber, Arkham appeared before all of them as Jester and began revealing his manipulations, while switching between Arkham and Jester. He told Vergil the reason his ritual had not worked was because it was incomplete and that the blood of a mortal priestess, who Sparda had sacrificed to seal the Temen-ni-gru, was required, blood which now flowed in Lady's veins. Arkham completed the ritual, but Vergil, Dante, and Lady seemingly managed to corner him. However since the tower had been unsealed, a column in the tower began to rise and, using the trio's surprise to his advantage, Arkham knocked them off. As the three are climbing to the top of the Temen-ni-gru, Arkham had reached the top of the tower and, after giving the seals their names again, ascended into the demon world. He eventually found Sparda's ancient sword, the Force Edge, and claimed his power, transforming into the dark knight's likeness.

A False God's Fall
After claiming the Dark Knight's power, Arkham confronted Dante after the half-demon followed him. His newfound power proved to much for him and he mutated into an enormous, misshapen blob creature. They began battling each other and eventually, Vergil joined the fight. Together the two brothers defeated Arkham and took away his newfound powers, sending him back to the human world. He landed, wounded and weak, on the top of the Temen-ni-gru, were Lady confronted her father. Arkham attempted to made one last ruse to save himself by plea for mercy by stating that he had done nothing wrong by attempting to become a god and that by sacrificing one human (his wife). Unfortunately for him, this failed as Lady, whom already knows his true evil self and is aware with the said ruse, shot him dead.

As Arkham
Though he looks rather harmless, Arkham is extremely agile and flexible as seen when he knocks Lady off the Temen-ni-gru and when he manages to disable Lady, Dante, and Vergil, even with two swords at his neck and a rocket launcher trained on him at point blank range, then knocking them away with a sweep kick.

As Jester
As Jester who is his clownish and psychotic alter ego, Arkham has a variety of magical abilities up his sleeve. One is his ability to disappear and reappear instantly, which he uses to avoid Dante's attacks, when the half-demon can't stand him anymore. Jester also has the ability to defy gravity, allowing him to stick to walls and ceilings. His other abilities as Jester are only seen in his boss battles in the special edition of Devil May Cry 3. He can summon spheres of electrical energy which he can direct in three different ways. Jester can also summon a large ball with a face, resembling the moon, which has a few abilities of its own. It can spit a large bomb out of its mouth, generate hands out of the floor that grab at enemies, create energy walls to make it harder for them to avoid Jester's energy spheres, and self-destruct. While on the ball, Jester (Arkham) always uses gestures like that of a circus ringmaster.

With Force Edge
After acquiring the Force Edge and mutating into a large blob, Arkham gained great strength, which he uses often, since he mostly swings wildly at his opponent with his multiple limbs. He can also send out tendril projectiles that home in on the target and create his Legions: sluggish, dolphin-like creatures that bear a slight resemblance to Sparda, which attack his enemies.

Trivia

 * Arkham's name and his alter ego, Jester, are most likely inspired by the Batman comics. Arkham's name comes from Arkham Asylum, the asylum where many of Batman's foes are imprisoned. Jester was originally going to be named Joker and his appearance and personality are similar to Batman's arch enemy, The Joker.
 * The name Arkham may have also come from the fictional city from the works of H.P. Lovecraft, for which Arkham Sanitarium was named.
 * Arkham's original name was going to be "Hyne", but Dante's voice actor, Reuben Langdon, thought that this would not be a good name for the English release, so he had Capcom change it.
 * The name "Hyne" is also the name of the enigmatic supernatural being called the "Great Hyne" from Final Fantasy VIII.
 * Arkham shares a similar trait with Hiruko Ubusuna, Ivo Shandor, and Queen Taramis, for each evildoer sought to sacrifice their relatives/descendants in exchange for ultimate power.
 * Arkham could be the true main antagonist in Devil May Cry 3 as he is also manipulating Vergil for his own gain. It's clear that even Vergil is against Arkham because he joins Dante's side twice in fighting him.