Damian Wayne revealed!

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On an unknown Earth (in the DC Universe) there existed Benders. People that could control the elements to their will and we're generally classified after each foundational element, with one exception. The 'Avatar' was the sole existence that mastered all four of those elements.

However, an avatar hasn't appeared since the last one of watertribe descent: Avatar Korra. No Avatar has appeared within earth tribe territory afterwards. For 200 years.
As a result, fights and wars have broken out regularly and caused for disharmony among all with no balance to prevent it.

The fact that benders slowly disappeared caused for even greater panic in the existing order and government.

Nowadays they were few and far between, shunned by society and even fewer revealed what they were in public. Regulations were strict and hard on them if they were to use their abilities.

Most of these regulations were neglected and had been abolished, not being of use anymore after the focus of the law shifted to something more immenent and dangerous:
The Justice League, metas and extraterrestrial threats.

These individuals that wielded powers akin to a God were a much realer threat than some fairytale that disappeared altogether decades into history. Still, every child has heard of the legend.
Even Diana, an amazonian princess who lived on a remote and secluded island far away from modern society, has heard of benders and how they were a dying race.

There was no hope of them being saved, sadly.

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Batman, as they found out, was one of the few surviving ones - an Earthbender that could control the sub-bending style referred to as metalbending. He kept both a secret outside of the manor and his nightly activities. Surprisingly even his protégés, as it turned out, were benders.

Nightwing was an Airbender.
It really fit him as he had this naturally easygoing atmosphere around him that he wore like a childhood blanket. It felt familiar since birth.

Red Hood was a Firebender and they were pretty sure he liked to do the explosions himself. He could even produce small bursts of electricity. Nothing major, but enough to taser someone or hijack an alarm system.
He loved giving people a shock in retribution when he was younger, especially on the streets where such a skill could temporarily paralyse others. Nightwing if he got too clingy too.

Red Robin was a Waterbender who'd always be calm in his deductions but could also carry the weight of a great typhoon if one managed to piss him off (Mulan reference intended).

The girls liked to train on their own, with Barbara actually having taught Tim her knowledge as his old, biological family had failed to do so.

The only exceptions in their family's ranks were Steph, Cass and the butler. They were regular non-benders but had the respect of each other.

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Now the 4th Robin doesn't exist at that time. Well, Damian Al Ghul exists, but he has refrained from finding and/ or contacting his father. He had run away from the Al Ghul's when it was revealed he was a worthless non-bender. He had inherited neither his father's great earth-/metalbending nor his mother's deadly water-/ bloodbending - he was deemed a failure to the noble Al Ghul bloodline.
He had learned every martial art related to the two bending styles, including their sub-styles, but neither had ever even hinted at showing itself. Neither had any other bending of the other elements. A non-bender.

As a last mercy from his mother, she helped him flee from the compound and sworn that, if he never showed himself in front of her again, she would not look for him.