He first appeared in Young Avengers #1 (April, 2005). The issue was scripted by Allan Heinberg and drawn by Jim Cheung.
Regularly bullied in school, Billy found solace outside the gates of the Avengers Mansion. One day, Billy encountered the Avenger the Scarlet Witch, who told him to stand his ground the next time it happens. She then touched his head, causing a red light to flash briefly. The flash heals Billy's wounds and a few weeks later he stands up for another kid being bullied, and demonstrates his powers for the first time, but is not fully prepared for their effects.
The full extent of his powers are unknown, though they include flight and lightning generation. He can cast spells using a technique learned from "self-help books" owned by fellow Young Avenger Hulkling's mother. By chanting a mantra/statement of intent (for example, "I want Iron Lad to forget") and focusing on the outcome he wants to achieve, he can cause different events and effects to happen.
Billy has said that Wanda Maximoff, the Scarlet Witch, was his favorite Avenger, and it has been implied that his powers are linked to hers. Billy's possible relationship to Wanda was revealed by the Super-Skrull and Vision.
In the past, Scarlet Witch was so desperate to have children that she used her reality warping powers to create twin sons out of fragments of the soul of the demon Mephisto. When the boys' souls were reabsorbed into Mephisto, because of the power Wanda put into the soul fragments, they destroyed him and their souls were reincarnated as Thomas Shepherd, the Young Avenger Speed, and Billy.
When the Runaways try to stay out of the conflict, they barely manage to escape from the government forces. Hearing of this development in the news, the Young Avengers decide to help the Runaways even though Captain America vetoes the plan. Wiccan uses his magic to locate and teleport to the Runaways. However, the Runaways believe that the Young Avengers have come to capture them and a fight ensues until Eli (Patriot) succeeds in convincing Nico to stop the hostilities. The two teams are later attacked by Noh-Varr, who captures Wiccan and Hulkling, and (apparently) kills the Runaway Xavin. The young heroes become the prisoners of the Warden, who proceeds to practice vivisection on the unconscious Teddy. The boys are rescued by Xavin, whose Skrull physiology allowed him to recover from Noh-Varr's attack. He comes close to killing the Warden, but Teddy stops him.
Wiccan is among the Secret Avengers who takes part in the first major battle of the Civil War and was one of the first two casualties. Tony Stark and Peter Parker realize that the two rebels with teleportation powers needed to be incapacitated to prevent an escape from the ambush. So, Wiccan and Cloak are shot with tranquilizer darts and rendered unconscious. This attack on two of his youngest followers infuriates Steve Rogers (Captain America). A violent battle ensues, by the end of which, Goliath lies dead, slain by the cyborg clone of Thor. Most of the Anti-Registration group escapes, but Wiccan is left behind and captured by the Pro-Registration side.
Billy and the other imprisoned anti-Registration heroes are eventually freed in a raid led by Captain America. This liberation is made possible largely by Teddy's shape-shifting ability, which allows him to mimic Hank Pym's retinal and voice patterns, and thereby release all the prisoners from their cells.
Billy is shown to be a member of the Initiative, along with Hulkling. However, it is later revealed that some, perhaps most, of the Young Avengers did not actually join The Initiative or even register. They were registered by their Earth-A counterparts; both Wiccan and Hulking are seen complaining about being registered to their "A Selves,. It's been subsequently shown that since registration, Billy has spent most of his time at home, apparently grounded, with his parents wanting him to register.
Billy must be able to hear his spells for them to be successful, but some of Billy's spells can be activated without words. He has consistently been able to create force fields, teleport others, and generate lightning and force blasts with little or no effort.
The Super-Skrull and the Vision's files consider Kaplan and Shepherd to be siblings. While Kaplan believes himself and Shepherd may be Wanda Maximoff's reincarnated children, Shepherd is more skeptical about the situation.
During their search for the Scarlet Witch, Wiccan is even more certain that he and Tommy are siblings. Both boys refer to one another as "brothers" and Speed claims to be the older of the two (though neither knows for certain) and also refer to the Scarlet Witch as "Mom" while searching for her.
Exchanges between Wiccan and Hulkling had led some readers to speculate that the two young men had a much more intimate bond than mere friendship. Allan Heinberg confirmed this speculation, stating that his intent was to reveal the relationship, and he was surprised that his subtle clues were picked up on so quickly.
Kate advises that Asgardian should change his codename to avoid the obvious puns ("ass-guardian" is implied, but not explicitly mentioned) when the press discovers that he's in a relationship with Hulkling, implying that the others already knew the two are gay. Kate, along with Stature, suggested the codename, "Wiccan," which Billy adopts.
When the team agrees to give an interview to Kat Farrell, and Jessica Jones warns Billy and Teddy that Farrell will probably ask if the rumors about them are true. After some deliberation, the two decide to tell her, with Teddy adding, "Why should Northstar have all the fun?
In Young Avengers Presents #3, Hulkling and Wiccan's romantic relationship is handled much more blatantly than previous issues. In it, they refer to one another as boyfriends. Speed also asks if their relationship is "getting hot and heavy" and jokes that Billy and Teddy would have a disastrous "mixed marriage" due to Teddy's alien heritage.
In a one-shot story about the final Avengers story, Billy and Tommy grew up as the children of the Vision and the Scarlet Witch. As a child, Billy witnessed the accidental murder of his mother by his uncle, Quicksilver. Her death eventually caused Billy (known as "Billy Maximoff") to become a villain, calling himself "the Grim Reaper." He and his brother Tommy communicated via holograms where Tommy implored his brother to give up his villainy, stating that he was apprehensive to fight him. Eventually the Reaper joined forces with Kang and Ultron-54 and their "final battle" with the Avengers. Billy appeared to be friends with this future's Oddball and the two discussed Billy's desire for revenge against his brother and father.
Billy and his brother were locked in battle until their father, the Vision, destroyed Ultron, ending the fight. As the Vision lay dying, he implored Billy not to give up on humanity the way he had when Wanda was killed. What exactly became of Billy following the Avengers' victory is unknown.
Unlike Earth-616, Tommy inherited their mother's mystical abilities and the nature of Billy's powers were unspecified. As the Grim Reaper, Billy wielded a scythe of unknown nature, either technological or mystic, that could teleport back to him if it left his hand. The scythe could presumably cut what he wanted it to while harmlessly phasing through what he did not, such as when he impaled Jessie Wingfoot without injuring Ultron-54. During the battle with Tommy, his scythe was able to deflect mystical force bolts conjured at him.