One of my biggest crushes in the MCU, I think it's difficult for many not to love Peggy Carter. The British soldier is more-or-less an American soldier in practice as you see her helping the Americans all the time. She is the first to notice Steve Rogers.
And helps Steve in his transformation to become a super-soldier and eventually our nation's hero Captain America. Along with that, she's very well capable at trying to stop Hydra.
Despite not really being a couple, it was pretty obvious how much Peggy and Steve liked each other.
Especially when Peggy catches a female soldier kissing Captain America.
So she volunteers to "test" Steve Roger's new shield. Good thing it was bullet proof.
Steve Rogers and Peggy Carter almost become a real couple but he "dies" in a plane crash, or so she thinks. She moves on, marries, and helps found SHIELD which is the main reason Captain America joins SHIELD.
The two are reunited before she dies of old age. I thought it was nice the MCU wrote that in, but it was a little odd seeing her so old. They gave Peggy Carter her own TV series Agent Carter, but I didn't get to really watch it before they canned it.