My KC flash has Divebomb, Low Gravity is in play, and I don't want the character I'm hitting to be moved for LOF purposes. (Hypothetical situation) Can I attack while directly above them, or is how does that work.

No. You cannot occupy the same square of any character on the board, no matter your level on the earhth. (Soaring or grounded.)

He would take the knockback no matter what.

Adding to that, while Flash as a flier can ignore character bases while he's moving (and even soar right over a character as part of his move) you can only make a HSS attack from a square in which you could legally end your movement. Since you couldn't end your movement in the same square, Flash cannot attack from that square and directly above. Consider having someone throw a smoke cloud up under the figure and then they'd be occupying hindering terrain and therefore not be subject to knockback per Low Gravity.