"Had enough," he growled viciously, "of life?" -- Ari, from The Angel Experiment
He eventually dies, but comes back later. He had wings retrofitted on by the whitecoats in book 2. In book 1, while fighting in the New York sewers, Max accidentally snaps Ari's neck and he supposedly died. However, at the School they fused the bones in his neck and brought him back to life, much to the dismay of the Flock.
Ari is the son of Jeb Batchelder, one of the top scientists at the School, and half brother of Maximum Ride. Unlike the other hybrids, Ari spent his early life as a non-mutant human. When he was three years old, Jeb disappeared along with The Flock, leaving Ari at the tender mercies of
the School's genetic scientists. The geneticists used Ari as an experiment, splicing his DNA with that of a wolf in order to create a new breed of Eraser that could live longer than the previous versions (those whose DNA had been spliced in the womb). But Ari didn't turn out as well as the scientists had hoped. He is always in a partially morphed state, not quite human, not quite wolf, (aside from several sightings), when first seen with Maya outside the diner where Max had ice-cream with Sam in School's Out--Forever) and says he had a rough transition period. He is often in pain, taking pain pills constantly in the second book. Angel informs Max that Ari loves her, loves her a lot, although he constantly tries to kill her. Max's reaction was something like: 'Jeez. Ari loved me? Like a little kid? Like a big Eraser? Was that why he kept trying to kill me? He needed to read an article about how to send clearer signals.' Ari wants Max to be his friend, although he tries to wound Fang to the point of hospitalization because he doesn't seem to like Max with Fang. He considers just taking Max as his friend, and the two will live together somewhere in hiding.
Ari is the most 'wolf-like' of the Erasers, always in a partially morphed state, never quite human, never quite wolf. One time, he was mistaken for the X-men character Wolverine by a young boy at Disney World. While he was talking to the kid, the kid asks for his autograph and he signs "Wolverine" with a flourish, but later in the series (School's Out Forever) he can't write his own name (as Max has to teach him).
During the climactic fight scene in The Angel Experiment, Ari fought, and had his neck broken by, Max. He was killed, but came back from the dead in order to kill Max, because of scientists at the School realigning his spine. However, the broken spine may have even been a blessing in disguise for him, as it allowed the School's scientists to graft a full pair of wings on his back. But it is later said that his wings don't work right and are very painful.
Ari and Max share many similarities, the most obvious being that they are both mutants and both hear "the Voice" in their heads. In The Angel Experiment, the possibility that Ari and Max are siblings is discussed.In School's Out Forever it is revealed that Ari has a strong (but reluctant) admiration for Max. He even comes up with a plan to steal Max away, so she could 'read to him and play games with him'. However his wolfish-seven-year-old self shows through when he adds that he planned to cut off her wings.
Ari's greatest desire is to have his father be proud of him, and is extremely and horribly jealous of how his father dotes on Max. He one time expresses a desire to choke his father until Jeb realizes he ought to pay more attention to him. His father is proud of him in School's Out Forever when Ari tells him of his plan to steal Max, however.
Ari may appear to be an adult in many ways because of his genetic engineering. In the first book, his traits seem to be quite adult-like, while in the second book, the reader discovers Ari's feelings and thoughts are often like those of his seven-year-old self. This is best shown when he goes to Best Mart in the second book, where he steals a Game Boy and is also disgusted by a woman holding a bra in public.
Ari also exhibits slight masochistic tendencies, as illustrated when he bites himself several times in School's Out Forever, because the pain "felt better."
Ari joined up with Max's half of the Flock in Saving the World and Other Extreme Sports. While the other Erasers were "retired" (killed), the white coats allowed Ari to continue to live, as his expiration date had recently appeared on the back of his neck, indicating that he'd die on his own anyways. After learning that his death is approaching, Ari becomes extremely depressed and seems to lose his hatred of Max and the Flock. When the Flock is held captive in the School, he shows Max some of the experiments and tells her of the white coats' plans for Re-Evolution, saying that even though he knows he will die soon, he hopes that the Flock can still escape and stop it. He helps Angel find Total (who the white coats took from the Flock) and breaks open a cage that they're trapped in. Realizing that the white coats will probably kill him for helping them escape, Max asks Ari to join them, much to the displeasure of the rest of the Flock (except, perhaps, for Angel). Ari is very grateful to Max for taking him in and promises to keep her safe. He joins Max's "mini-flock" when the Flock splits and goes with her, Angel, Nudge, and Total to Europe. During this time, Max notes that he doesn't talk much, as if he didn't think he had a right to an opinion. It is revealed that, because Jeb is Max's father, Ari is her half-brother. Ari tries to back up Max during her tests against Omega; and later, when a mutant riot breaks out, he uses his strength to literally "toss smaller mutants into the mosh pit of death". In the midst of the fight, he suddenly collapses. Max tries to find where he was hurt, but he wasn't injured. His time has run out and he suddenly expires.
Later, there is a memorial held in his honor by the Flock and Jeb is at Ari's make-shift funeral. Each of the flock drops dirt onto the casket, and Angel, the Gasman, Fang say a few words. Most of the Flock -- primarily Max, Nudge, Total, and Angel - broke down and cried.
After his death many villians had brought Ari back to intimidate Max, such as Mr. Chu who had dug up Ari's body and combined it with robotic parts to intimidate Max but he had utterly failed -- due to one of Gazzy and Iggy's bombs.
In Nevermore, Ari was cloned perfectly (retaining all of his memories) and had killed Maya but was then killed by Dylan when he attempted to kill Fang. This was the last Ari was seen in the series, though Jeb's survival and Angel's words suggest that Ari was recreated again after the book ended.
- Ari also had a Voice similar to Max's, as shown in School's Out Forever.
- He once went to Disneyworld where he thought by one child to be Wolverine from X-men and signed a shirt as "Wolverine", although later in the series it is said he can't even write his own name.
- Ari has a very strong admiration for Max, he wants their relationship to be like a normal brother and sister's, he wants Max to read to him and teach him things, although he also wants to rip out her wings.
- Ari is shown to come back to life multiple times, all of them because people are trying to get to Max by bringing her little brother back, or attempting to have him kill her.
- Ari is one of the more wolfish Erasers, as the others being stunningly attractive Ari seems to be stuck between Human and Wolf.
- He is shown as taking many pain-killers to ease the pain of wings, although he was willing to get the wings no matter if it hurt or not.
- Ari's birthday is somewhere in April. However, he may have never turned eight due to the reason he died shortly afterwards in Saving the World and Other Extreme Sports.