Peggy Carter met Charlotte Rogers at a military base, the same one Charlotte's brother, Steve, was recruited to.
Both of the girls were best friends and Agents at the SSR. During the war, they actually had some respect.
Peggy fell in love with Steve. His closest friends- Charlotte, Peggy, and Howard Stark were all very sad over the loss of the best person they knew.
They have a hard time getting over it, and things get worse when Howard is accused of selling weapons to US enemies.
Meanwhile, Peggy doesn't know if she will fall in love again, and Charlotte feels incapable of it.