Spider-Man fans may be able to name most of the big villains that the web-slinger has gone up against over the years, but there are still some surprises up Spidey's sleeve: like Teresa Parker, his sister. Peter Parker has a legitimate sister, which is a lesser-known fact. She will be returning soon, so it is a good time to brush up on exactly who she is.
Teresa Parker was first introduced in the original graphic novel, Amazing Spider-Man: Family Business back in 2013. Written by Mark Waid and James Robinson with art by Gabriele Dell'Otto and Werther Dell'Edera, the story created a new wrinkle in the family history of the Parkers. Teresa showed up when the hero was attacked by armed gunmen inside his apartment - not as Spider-Man, but as Peter Parker. They attempted to kidnap him, but he got away, just to land on a car driven by Teresa who picked him up and revealed that she was his sister.
By the end of this graphic novel, Kingpin claimed that he used Mentallo's abilities to make Teresa believe she was a Parker. He also adjusted Peter's mind to believe that Teresa and he looked legitimately alike. Here it was revealed that Teresa's last name was Durand. Peter decided that he liked having a sister and that even if they weren't connected by blood, they could still be siblings to one another - but that wasn't the end of the question of Teresa's identity.
Teresa reappeared in Peter Parker: The Spectacular Spider-Man - written by Chip Zdarsky with art by Adam Kubert and Juan Frigeri - on the run from S.H.I.E.L.D. While Peter tried to protect her and uncover plots by the Tinkerer, events escalated until time travel became necessary. In an alternate past that would not affect their timeline's future, Teresa did some independent digging - seeking answers that she needed to know. Despite being told that she was not a Parker, she questioned if it could be true and after contacting Nick Fury, she found out she really was the Parkers' daughter. So yes, Spider-Man has a legitimate, honest-to-goodness sister by blood relation.
As revealed in Amazing Spider-Man: Family Business, Richard and Mary Parker - Peter and Teresa's parents - were C.I.A. agents. In short, they were spies who held secret lives of their own. Peter was their firstborn son and they planned to retire, but it wasn't meant to be. They had Teresa and fell in the line of duty, but why she was never fully connected with her family remains a mystery. The Parker parents' history as legitimate spies was clearly a well-guarded secret that took a long time to come out. They had safehouses, DNA-protected secrets, and succeeded in hiding their true identities much better than Peter did.
Teresa Parker - as the original graphic novel suggests - got into the family business, becoming a CIA agent. Spider-Man's sister is a bona fide spy with S.H.I.E.L.D. connections. She faces danger just like her brother does, but with less of his moral fiber to a certain extent. Teresa isn't afraid to have a few casualties if the job gets done, which has been known to cause conflict when fighting by Peter's side. Her most recent appearance was in Nick Spencer's run of The Amazing Spider-Man - specifically with the Symkarian Plot arc. She was involved with her spy partner, who was captured, tortured, and ultimately killed by the Chameleon. She spent the majority of this arc chasing him down, but she isn't done with him yet.
There are already plans for Spidey's sister to return in the upcoming Chameleon Conspiracy arc of The Amazing Spider-Man. Though she hasn't had many appearances just yet, Teresa is a character worth keeping an eye on. She has been very supportive of her brother and his role as Spider-Man, joining him in battle and proving herself to be a hero in her own right - even if she doesn't currently have a codename. Currently, Aunt May does not know that Peter has a sister, which maybe Teresa Parker's next appearance will tackle so the Parker family can be fully reunited.