WARNING: The following contains spoilers for The Seven Deadly Sins: Dragon's Judgment Part 2, now streaming on Netflix.
The (former) leader of the Ten Commandments and Commandment of Piety, Zeldris has had an interesting journey in The Seven Deadly Sins. The younger brother of Meliodas and fellow son of the Demon King, Zeldris is one of the more tragic characters in the series. Zeldris follows his heart surprisingly often for a Demon Prince, and like his brother, he has suffered a great deal for it. As his backstory and role in the series continue to expand, it becomes clear that he is just as much a victim of the Demon King as anyone else.
When Zeldris and Meliodas served together in the Ten Commandments 3,000 years ago, Zeldris idolized his big brother. When Meliodas confesses his intent to abandon the Demon Realm to be with Elizabeth, Zeldris is crushed, despite longing to do the same with his love, Gelda. Too afraid of the Demon King to flee, Zeldris lashes out at Meliodas instead, declaring they are no longer brothers. The series and even the Seven Deadly Sins themselves would look very different if Zeldris had gathered the courage to go with Meliodas.
Staying turned out to be the worst thing Zeldris could have done, as he was soon ordered to execute the Vampire Clan, including his lover, Gelda. Rather than follow the Demon King's order, Zeldris secretly sealed away the Vampires instead, Gelda included. Hating Meliodas for fleeing with his love while Zeldris was forced to imprison his own, he spent the next 3,000 years as leader of the Ten Commandments, driven by his desire for revenge on Meliodas. Zeldris' relationship with his brother and Gelda's whereabouts and status being used against him left him tormented and conflicted.
After he was finally released from the Demon Realm after being sealed by the Goddess Clan, Zeldris learned that the Vampires had been freed, discovered by the Sins, and wiped out. Zeldris carried out his duties as leader of the Ten Commandments without a hint of remorse until Meliodas informed him that he had sealed Gelda away and would free her if Zeldris helped him gather the Commandments and become the new Demon King. Just like his brother, Zeldris drops everything and jumps at the chance to reunite with Gelda, working alongside Meliodas and even giving up his Commandment.
When Zeldris is later forced to consume the Commandments against his will, knowing it will make him the Demon King's vessel, his father masquerades as Gelda to keep him subdued, and when Zeldris realizes it, the Demon King tells him that Meliodas lied about sealing Gelda. To recap: at this point, Zeldris has been ordered to kill Gelda, sealed her away, told that she was dead, told that she was alive, manipulated by a fake copy of her and told that she was dead again. It's a major feat that Zeldris remained as relatively sane as he did with so much back and forth regarding the woman he loves.
Zeldris has only ever wanted to be true brothers with Meliodas and live freely with Gelda, spending 3,000 years believing neither was possible. Even after multiple betrayals and deaths because of Meliodas' actions, there's still a part of Zeldris that wants to be brothers again. A far cry from the Commandment of Piety who fought Meliodas to the death and watched him die, Zeldris' devotion to love proves that it could have just as easily been him who lead the Seven Deadly Sins.