I've never heard of Celeste Wallander, testifying now in a House Committee; according to this RedState article, she was apparently instrumental in getting the phony Muller investigation started, and is some kind of [criminal] associate of Fiona Hill.
https://redstate.com/stu-in-sd/2021/07/13/who-was-the-leaker-who-fingered-general-flynn-n409487
Smearing President Trump as a 'Russian asset' is something Wallander was pushing hard:
"On March 8th, before [Fiona] Hill was scheduled to meet with her staff for the first time, she had breakfast with Celeste Wallander, at the Blue Duck Tavern, near Georgetown. Wallander had worked as Obama’s White House adviser on Russia, and she and Hill had crossed paths for more than twenty years. There was clear evidence that Trump and members of his circle had coordinated with the Russians, Wallander said. Trump’s recent attack on NATO as being “obsolete” showed that he intended to do whatever Putin wanted."
https://www.newyorker.com/magazine/2020/06/29/what-fiona-hill-learned-in-the-white-house