Lovable nerd Black Cat.
In the role of a superhero, maintaining your secret identity is crucial to keeping your loved ones safe from your enemies. Because of that, it is necessary to keep the two parts of your life as separate as possible. For example, while Peter Parker is dating Mary Jane, people can't also see Spider-Man dating her or someone will figure out his secret identity. Most of the women that Peter has dated were individuals that he met and started seeing as Peter Parker. If they found out that he was really Spider-Man, it was only after they had already started seeing Peter.
The one exception to that rule is Felicia Hardy aka the Black Cat. She first encountered Spider-Man in 1979's The Amazing Spider-Man #194 by Marv Wolfman and Keith Pollard. Felicia was raised by her father to be one of the best thieves in the world, which means running into Spider-Man frequently. Upon meeting Spidey she is instantly attracted to him and eventually confesses her feelings for Spider-Man in The Amazing Spider-Man #226 by Roger Stern and John Romita. At this point, Felicia has never met Peter Parker and is only interested in Spider-Man. Peter starts to return the Black Cat's feelings after she turns her life around and starts to fight crime with him. During this brief period they swing through New York side by side throughout the night. The Black Cat is the only love interest who wants to be with Spider-Man and not with Peter Parker.
Peter is so relieved to share this part of his life with someone and thus his feelings for her grow deeper. He decides that he wants Felicia to share in every part of his life so he reveals to her that he is really Peter Parker in Peter Parker, The Spectacular Spider-Man #87 by Bill Mantlo and Al Milgrom. The Black Cat is actually horrified to find out that Spider-Man is really Peter Parker. She can't believe that her precious Spider-Man could be some ordinary person behind his mask. Felicia even begs him to put his mask back on and be her "Spider." She says that he should live in a glamorous Spider-Cave and not a cheap, regular apartment. Felicia is able to get past those initial impressions and come to accept him for who has is. While Peter goes about his regular life in the morning, his nightlife as Spider-Man is saved for the Black Cat.
They eventually break up as the secrets between them grow. But Spider-Man hasn't been able to share that side of his life so completely as he could with Felicia Hardy. Peter's other partners were mostly part of his civilian life while the Black Cat was Spider-Man's only girlfriend. That bond has kept them close as friends even as Peter Parker's relationship with Mary Jane has grown.
