Natasha knew him from red room
He was taken by the red room at the age of 8 when they tried to kidnap his younger sister, he attacked the agents and his mutant genge activated killing the agents and accidentally his family as well by boiling their blood.
His power is Hydrokinesis, allowing him to control water in all of its various forms including ice and blood. (he uses it like water benders from atla)
He faints from the fear, horror and exhaustion.
The red room make an exception and take him instead of just killing him wanting to use his powers for themselves.
When he tries to escape they put a electric collar on them that shocks him whenever he tries to his powers or when the agents press a button on their control devices.
By the time he is in his teens he is one of the best fighters in the red room without using his powers. And using his powers he can't be beat.
He was discovered healing Natasha and was punished before they forced him to heal agents and victims so they could torture them more. And if he refused he was tortured as well.
He falls in love with Natasha but when they are caught kissing they took him to another base where they used a device like the one hydra uses for the winter soldier to erase his memories of everything.
And they manipulate Natasha by making her kill a hooded man and when they told her to remove the hood it was her love.
They had used tech and surgery on a random man to make him look like him. It broke Natasha even more thinking she had killed the man she loved.
The red room use him for their assassination by making him uses his powers so most of the killings looked like accidental drowning.
For years he was the red rooms best assassin as he had no feelings, ideas or humanity he was just a empty tool.
By the time he was almost 30 he had killed hundreds of people.
S.H.E.I.L.D knew of this assassin but was never even able to find out what he looked like.
So fury assigned the Avengers the Misson of catching the assassin.
They manage to work out his next probable Target. Tony goes off on his own to corner him but he fights back and he is quickly loosing so he calls for back up.
Steve, Clint arrive but are still loosing as he is a skilled fighter and they can't fight water and ice.
Natasha arrives after having stopped agents that for some reason were only focused on her.
She attacks him using the element of surprise, he attacks her back pinning her down while the rest are pinned with ice. He puts a knife to her throat as the team screams her name.
He goes to kill her but he gets a good look at her face, eyes and for some reason he can't bring the knife closer. His arm starts shaking but his body won't obey him
She manages to get out from under him and uses a widow bite to stun him before Steve manages to break free form the ice and knocks him out with his shield.
They take him back to SHEILD to be interrogated.
Natasha brings up that he could have killed her easily but something seemingly stopped him.
Agents remove his gear and mask while he is unconscious while they chain him up.
The team is called to watch the interrogation and Natasha gets the shock of her life when she sees that the assassin is the man she loved and though she had killed.