Betty Ross went to work for S.H.I.E.L.D. as one of their head scientists studying the Super Soldier Serum originally used on Captain America; during that time she met fellow researcher Bruce Banner and the two began a romantic relationship. The relationship ended however, when Bruce decided to test an experimental version of the serum on himself and it went wrong causing him to became a creature of raw power called the 'Hulk'. Despite this the Hulk retained some of Bruce's memories and still loved Betty, but then Hulk was eventually stopped by S.H.I.E.l.D., after causing massive damage to New York City.
After that' Bruce was put on daily tranquilizers and kept on watch to control the transformations which also lead to Bruce and Betty's relationship becoming strained.
Betty then traveled with Nick Fury to the Atlantic ocean to search for the body of Captain America himself after finding clues to his whereabouts; once they found him and returned to New York, she managed to get Bruce to come help her examine the body but Cap soon revived and fought his way through SHIELD's defenses. Once he calmed down, Betty & Nick Fury then helped Captain America joined the newly formed Avengers as Fury explained to them about the alien Chitauri.
During their first mission Betty and Fury found out that Bruce wasn't trying to help them make another Super Soldier formula like last time, instead he was just trying to improve the Hulk serum so he could control the transformation on his own. Fury tried to stop Bruce, but got distracted while dealing with the Chitauri invasion, after which Bruce became the Hulk again using his new formula and was able to control it this time during the attack. He saved Betty in the process and attacked the aliens; the other Avengers arrived and fought the aliens off as well. During that time however, Bruce lost control as the Hulk again' and attacked the other heroes. Betty was able to secure a tranquilizer just in case and gave it to Black Widow who used it on the Hulk, Betty was then able to calm down Hulk until he transformed back into Bruce.
Bruce was then contained and sedated by SHIELD and put in cell, causing Betty to become distraught. In the following months Bruce demanded to see Betty but his jealous & resentful jailer Dr. Oiler refused.
Betty was then tasked with studying the remains of the alien ship the Chitauri where in; she quickly discovered that the vibranium hull it was made of had turned incredibly brittle after Bruce attacked it as The Hulk. She then finally saw Bruce and told him about her discovery and they both compared notes helping rebuild their relationship to.
She then found that the vibranium cube that was on the spacecraft contained an enormous amount of kinetic energy.
After the Avengers' failed mission to Wakanda to find more Vibranium, Betty went back to Bruce again to see if he anymore to go on; Bruce revealed that he discovered the aliens' weakness, gamma radiation made the vibranium brittle when the Hulk struck it with his fists, after this revelation' Betty then built a gamma generator to use against the aliens technology.
As the field created by the Chitauri Command Ship reached New York in another invasion attempt, Betty finished the generator and together with Wasp, she & her evaded the aliens while fighting them off and eventually made their way to Bruce, who was dying from an overdose of gas by a treacherous Dr. Oiler. Bruce apparently died on the other side of the glass, but not before he & Betty confessed their love to each other, after that Betty & Wasp left to help the other Avengers fight the aliens.
The two headed to Wakanda where Betty gave the generator to Iron Man, Iron Man used the device to destroy the Chitauri command ship and bring an end to their invasion. Betty and the others then watched on as Giant Man peacefully died in Wasp's arms after the aliens attack.
The team then headed back to New York to the SHIELD base which had been left in ruins, as Betty walked through the wreckage with Wasp and the other Avengers' she thought she heard Hulk's voice calling to her silently, she went to the cell that Bruce was in again and found that Bruce had managed to survive the poison gas attack by transforming into the Hulk once more and escaped.
During Giant Man's funeral Betty comforted Janet for her loss, and in return Janet reassured Betty that while Bruce may be on the run now at least he was alive & safe.