WARNING: The following contains spoilers for You Look Like Death: Tales from the Umbrella Academy #1-2 by Gerard Way, Shaun Simon, I.N.J. Culbard, and Nate Piekos of Blambot, on sale now.
When a new academy debuted at the end of The Umbrella Academy: Hotel Oblivion, it became clear there were more super-powered kids than initially believed. The thought was that Sir Reginald Hargreeves acquired the last remaining seven survivors from a group of 43 babies born that special day, but the door was blown wide open by Mother unveiling this new crew to Luther and Co.
Then in the second season of the Netflix series, the official word was that they were called the Sparrows when the family got back from the timestream. Now, while there hasn't been any origin story in either adventure thus far, that veil might have been slightly lifted in the two issues of Tales from the Umbrella Academy: You Look Like Death so far.
There are secret agents other than Abhijat working with Reg at the family mansion. They're monitoring a junkie Klaus, ensuring he doesn't sneak back in after he got kicked out, and shadowing Reg in all his activities. He deems them part of a mysterious "Company," and one's even found in training sessions as he forces Klaus to contact the dead in a cemetery. This same one aids him in electroshock therapy to push Klaus' abilities further so it's safe to assume they might have been helping him train other kids. That means the Company could have had access to Reg's scientific research, weapons, and maybe even helped him scour the globe for these kids. It's hard to see him using the Televator to get to all 43, and this flashback suggests he had trusted help outside of Pogo and Mother. He was part of an inner-circle in the show called the Majestic 12, but he clearly didn't regard them as close allies, just pawns. The Company, though, is something he has an affinity and loyalty to, with these masked agents feeling like cultists.
So, if the Company recruited kids for themselves or used genetic experiments, splicing and cloning them to be perfect weapons, it'd explain the Sparrows from Hotel Oblivion. Mother knows about them so she may have been privy to the Company's dealings and this back-up team, which would explain why she's ready to trust them to either work with Luther's group or even replace them as the newbies teleport in from their Norway base.
Hopefully, the miniseries will explore this further in-depth because again, the thought was that Reg was a loner. If he's bought into the Company, chances are they have just as many secrets as Reg and would be thinking on the same page.
The Company might even have a different vision and intention than upkeeping Reg's legacy, but more will be revealed once more insight is provided into all these operatives.