Ben 10 has several characters possessing their own arch-enemies:
Ben Tennyson
- Ben has many contenders to the title, but it varies greatly in how Ben treats them; by the end of Ultimate Alien, he treats the likes of Animo and Albedo more or less as persistent but lower-tier villains, with the likes of Vilgax and Zs'Skayr being seen as impressive challenges for his ego to overcome, but Ben never extends such light-heartedness abandonment to Kevin and Malware.
- Ben's greatest enemy of all is Vilgax, by virtue of being by far the oldest and most persistent, as well as having a direct connection to Ben's Origin Story, and appears in all series as an antagonist, which extends to the reboot, where he is treated as the most serious and dangerous threat to him of all. In fact, even when faced with the Eldritch Abomination Dagon, Ben's main concern was Vilgax (who had been demoted to Dagon's herald but eventually usurped him and absorbed his powers). Their rivaly proved so strong that Vilgax once hatched a plot to wipe out every version of Ben from the multiverse just to get back at him, with the resulting conquest protentional being a bonus to him.
- Ben's most recurring human enemy was Dr. Aloysius Animo, who ended up overshadowed by more dangerous foes by Alien Force, but would remain a thorn in Ben's side from his pre-teen heroics until at least his 40s. In fact, when getting the Negative 10 to start infighting, Ben shows an uncharacteristic Worthy Opponent feeling toward Animo by feeling he should be "next in charge" of the group after the Forever King, apparently just because he's his next biggest enemy in the group.
- Kevin 11 served as Ben's antithesis in their childhood, in the way he used his powers for his selfish desires and eventually mutated into a amalgamated form of Ben's original ten aliens, but made a Heel–Face Turn when they became teenagers, becoming best friends, with Ben even calling Kevin the brother he never had by "Ultimate Alien". He was replaced as Ben's antithesis by Albedo, Azmuth's former assistant trapped in the form of a color-inverted version of Ben, who is treated as more of a Harmless Villain than Kevin was, though still attempting to prove his worth by the manner of being the creator of the Ultimatrix. When Kevin went insane again as "Ultimate Kevin" after being mutated by the Ultimatrix, Ben more or less spent the entire arc in crisis mode and was a lot quicker to use extreme tactics to take Kevin down than his other enemies, not just because a power enemy was on the attack, but because Ultimate Kevin was a personal failing he needed to correct.
- Zs'Skayr was the consciousness hidden within Ben's Ghostfreak transformation, serving as an Enemy Without to Ben, even making afraid of the Ghostfreak transformation both before and after his escape, with the general intensity of their battles often putting Ben at the end of his rope, to the point he formed an Enemy Mine with Vilgax to confront Zs'Skayr, and even then only won by trapping him back in the Omnitrix. By Omniverse, Ben, in a deviation from his battle happy persona, was content to leave Zs'Skayr be on the Anur System rather than fight him until her had no choice.
- The rogue Galvanic Mechamorph Malware would serve as another personal adversary for Ben, with Ben even fearing him due to him destroying Feedback, essentially killing a part of Ben and being implied to be the reason he removed the Omnitrix in the Time Skip between series. Indeed, despite being restricted to the lighter Omniverse series, neither Ben nor the narrative ever treated Malware or the threat he poses as a joke, and his effect on Ben proved long lasting enough for Ben to freeze in terror at the mere sight of Malware.
Gwen Tennyson
- Gwen started out with Hex, the villain of her A Day in the Limelight episode that began her connection to magic with the Charms of Bezel, with his returning episode making it clear he desires revenge on "Lucky Girl" for his previous defeat and his final episode having him blame her alone for his downfall, completely overlooking Ben's part in his defeats, making him the only villain in the show to disregard Ben. During the Time Skip, by the events of Ultimate Alien, Hex fell into a depression that made him unwilling to fight Gwen even when she invaded his house, instead just asking her to leave him alone. He ends up undergoing a Heel–Face Turn by Omniverse, becoming a teacher at Gwen's university and the pair manage to bury the hatchet, with Gwen even calling Hex her favourite teacher.
- However, Gwen's biggest personal enemy was Hex's niece, Charmcaster, being more directly linked to her magical journey when Gwen stole her spell book during her second appearance, leading Charmcaster to hold a grudge over Gwen for being sorceress, with Gwen being her main target during the Negative 10 saga. When she returned for Alien Force, Charmcaster's goal was to absorb Gwen's mana, and steal her boyfriend in an Evil Is Petty move, with an alternate timeline visited in one episode mentioning Charmcaster as having killed Gwen. During Ultimate Alien, Charmcaster starts going through a Heel–Face Revolving Door, with Gwen trying to befriend her to redeem her, which continues into Omniverse, where Gwen was Put on a Bus, but still showed in Charmcaster's episodes.
Kevin Levin
- While Ragnarok appeared for only one episode, his link to Kevin's origin is enough to qualify as Kevin's arch-enemy, especially since he killed Kevin's father, thus robbing Kevin of the parental guidance that could of prevented his life of crime. However, the retconning of Kevin's past in Omniverse devalues Ragnarok and his murder of Kevin's father.
- In Ultimate Alien, Kevin takes a significant interest in stopping Aggregor, a fellow Osmosian trying to obtain the power of a baby Celestialsapien to gain omnipotence, seeing Aggregor's power quest as a reminder of his Drunk On The Darkside antics as Kevin 11. Ultimately, Kevin subjects himself to another amalgamation mutation to stop Aggregor by becoming Ultimate Kevin, leading to his Arc Villain saga that ended the second season.
- In Omniverse, Kevin gains a new nemeses in Rooter Proctor Servantis, another villain with major ties to Kevin's backstory and who implanted false memories into Kevin and the Plumbers' kids to get them close to Ben to destroy him, with the aforementioned Ragnarok ultimately turning out to be one of Servantis' allies and Word of God saying that Aggregor was one of Servantis' pawns. Holding power over him like no villain before, Servantis' ultimate goal was to have Kevin kill Ben in his paranoid belief that Ben having the Omnitrix heralded a "coming storm" that would destroy the universe.
Rook Blonko
- Rook has his former master Kundo, who hates Rook's use of technology and breaking of old Revonnahgander traditions by leaving Earth. Their conflict becomes truly personal when Rook finds out Kundo had injured his father while trying to stop his sister leaving Revonnah to join the Plumbers and then hunted her and their youngest brother to Earth. Kundo also serves as the villain in the episode that ends Rook's Character Development and his defeat of Kundo culminates in Rook finally earning the rank of Plumber Magister.
- Rook also has Sitcom Arch-Nemesis relationships with Pax and the Vreedle Brothers, with the former and his hypocritical environmentalist beliefs being the only one in the series to irritate Rook, and the latters' destruction of the Plumber Academy forcing Rook to complete his finals on an asteroid outpost, which he says was not an enjoyable experience, and resulted in him holding a grudge against them, though they don't see the big deal.
Other Characters
- After the events of "Dr. Animo and the Mutant Ray", Grandpa Max has a subtly rivalry with Dr. Animo, taking the Doctor's threat more seriously after being mutated by him, even getting noticeably violent towards Animo specifically during the Negative 10 saga. During Alien Force, the two have a private war in the Null Void as the Wrench and D'Void. Omniverse even shows that the two will be at odds, though more as Friendly Enemies, in the future, with Animo going from giving Max a sincere farewell at his retirement party to trying to kill him when the opportunity arises, though Max takes both in his stride.
- Anti-Villain King George from the Forever Knights had Dagon. The Forever Knights' ideology towards persecuting aliens is rooted in their conflict with Dagon (even chronicled in history as Saint George and the Dragon), and their ultimate mission is to prevent Dagon's return.
- Professor Paradox has fellow time traveler Eon, a rogue alternate version of Ben who has sought many times to conquer or destroy time, even stealing Paradox's Chrono-Navigator at one point in an attempt to achieve this.
