Damian Wayne revealed!

K.F. returns with Who?

It was during Tim's time as the Young Justice's Robin.

He still questions most of the things he had bore witness to at the time but the undeniable truth was right in front of him now. As well as a matching DNA test he had, like Bruce, conducted himself thrice to validate the person.

Truth be told, Dick, Tim and Bruce had heard of Damian's existence before meeting him a few months prior and the difference in personality was striking to both Bruce and the time they'd met him before Talia came to drop him off with them.

How it happened? you may ask.
Well, sit or stand and read on, at whichever time you may be doing so.

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One day in Mt. Justice where he was relaxing in the lounge, working on his computer with an energy drink, an earthquake had suddenly shook the entire mountain. In the process his computer got fried by the can toppling over it, ruining not only his lifeblood but also his best friend, the computer.

He cursed, going to investigate the quake's origin - after all  Mt. Justice was safe against most natural disasters and had a lockdown mode. They could be hit with a Point 10 Earthquake or Tsunami and the place would still be standing.
He also doubted it had been any of the others as this was their agreed-upon chill-time.
If anything, he'd have heard the oven explode before an earthquake occured.
Though that statement would be invalid if he were in Gotham, where everything was possible.

Arriving near the main area he found something that distinctly reminded him of the time Impulse had arrived: An electromagnetic energy field, whose power was being discharged into the surrounding area as a means to stabilise itself in space-time, was there.

He called the others over the intercom system to greet the invader and possible second time traveler. They gathered within the next 3-7 minutes.
Just as most had arrived the lightning bolts gathered and struck one spot, a cloud of dust and light blinding them.

"-o, is this your correct timeline?" drawled a deep, baritone voice that not only sounded tired and annoyed but also had an eerie resemblance to Bruce's I could be doing 20 other things but I'm stuck surrounded by Idiots growl.
It had him instinctively straighten up.

The voice sounded like him too, Tim noted at the same time.

"I think so... we're in Mt. Justice and it looks just like when I had lived here" intoned a decidedly much younger and more excited sounding male. This one also reminded the team of someone, though not of a Leaguer but someone from the past. A dear yet painful memory echoed in the heads of the original young Justice team, making them queasy.

Nightwing gasped, a premonition appearing in his stomach as heavy as rocks. It couldn't be.. right?

The dust cover settled and there were two people. The team tensed.

Their eyes darted to the bigger threat: the undoubtedly older one with the deeper voice, who was not just tall as all hell but also dressed in a black suit (which had an uncanny resemblance to Batman's, several members noted.)
And the overall resemblance didn't just end with the suit; his built, facial structure, even his stance overlapped with Batman's, which Dick and Tim noticed almost immediately.
The two bats looked at each other in trepidation. A clone?

The other was

".... Wally?" Dick choked up on his saliva, tears brimming in his bright blue eyes. Tim gasped, then looked between the groups, clearly shocked.

"Dick!!..." The boy screamed, jovial, as he looked towards the speaker. His face drew a blank. "Wait, who are you?"

He looked between Robin and the adolescent, confused at the discrepancy in voice, costume and person.