Origin
Heather Sante was a S.H.I.E.L.D. agent with a unique amount of experience dealing with alternate realities, time travel and alien beings. She even fought a zombie Spider-Man once. Her special qualifications got her tasked by Director Tony Stark to deal with the new Captain Marvel situation. The belief was that Mar-Vell had come forward in time and needed to be monitored to ensure he didn’t die in the present, which would alter the past.
Creation
Heather Sante was created by Brian Reed and Lee Weeks for the Captain Marvel limited series that followed up on Mar-Vell’s apparent resurrection during Civil War.
Major Story Arcs
The Initiative
Her investigation didn’t give her any answers beyond exposing how deep Mar-Vell’s obsession with the painting ran. Talking with Stark, it was decided that she should start investigating the world around Mar-Vell, starting with the Church of Hala that he recently been founded. She went undercover and joined the church’s membership. While skulking around, she overheard some of the legit members being panicked and talking about a member who had died in a fall. She snuck past to see for herself and found the dead member of the church laid out on a table. In death, the man had reverted into a Skrull.
This discovery took Agent Sante’s investigation in a new direction. She found records on the Skrull corpse that revealed it had been the Skrulls behind the series of attacks on Mar-Vell. Using this new information, she located a hidden Skrull facility and spent three days going through all the information she found available. This was how she learned the truth about Mar-Vell. He was not Mar-Vell. He was Khn’nr, a Skrull volunteer who had been genetically and psychologically modified to be a perfect impersonation of Mar-Vell. The process turning into this had gone wrong, and he escaped. While she was there, Mar-Vell — no, Khn’nr — returned via a Negative Zone portal. His return to this place trigger the memories of his escape and what he truly was. She confronted him, calling him a liar and a malfunctioning weapon.
Khn’nr convinced her that he was choosing to be the hero Mar-Vell over an invading Skrull. He told her he could have killed her already if he wanted and that if she truly did not believe him then she should shoot him. She chose to believe him.
Secret Invasion
For further details: Secret Invasion
She later returned to find Khn’nr gone and a letter left for her. In it, Khn’nr apologized for leaving and doing what he felt he had to do. He was no longer sure he could stop the Skrull invasion, so he had left to do what little good he could do while the invasion happened. She didn’t see him again as he died in the process.
It is unknown what happened to Agent Sante after S.H.I.E.L.D. was dismantled and whether she became an agent of H.A.M.M.E.R. as many others did.