Unlike the Hargreeves siblings of The Umbrella Academy who discovered their powers from a young age, Harlan obtained their abilities in a unique way.
Through the absorption of sound, Harlan could project energy to manipulate or repel objects, a power associated with Diego. It displays an amplified version of Vanya’s powers when she loses control in The Umbrella Academy.
Harlan unleashed a continuous blast of sonic energy that formed a kind of energy shield, and was trapped in the center. Although Vanya hadn’t mastered his own power at this point but Harlan seemed like he did at The Umbrella Academy.
Despite Vanya’s actions in taking a great burden off Harlan, she was unable to completely remove his powers at The Umbrella Academy. As Harlan and her mother drive into their new life, she is shown twirling a toy over her hand.
This revealed that Harlan had retained her telekinetic abilities and also, crucially, that she could use them with minimal effort, and that might not be all at The Umbrella Academy.
If Harlan was able to keep some of her powers, she might unlock something stronger in the future. And without Vanya in the equation, Harlan could become a major threat in The Umbrella Academy season 3 and beyond.