I personally find Pathfinder to be the friendliest to new players, but it's honestly going to depend on what you are most comfortable with teaching. I learned with 2, and know plenty of women that started with everything from 1 through 4. I would also suggest not getting too rules-crazy at first if it seems to frustrate her (THAC0, ugh). I'm not clear on this so feel free to clarify if you are hoping to introduce her to an existing group or form one with her in it, but my advice in this post is tailored to solo campaigning. if you are doing a solo campaign to ease her into playing then your focus should be on asking her what she wants. Character creation can unfold over a fun evening or two by asking a lot of questions about what sort of game she wants to play, what sort of character she is interested in, building backstory/friends/enemies/goals, or if she doesn't find that fun then you can do anything from a quick character generation to offering her a few pre-made options and letting her customise from there.
Frankly when I started DMing a solo campaign for my husband (and he in turn ran one for me) one of the most enjoyable parts was seeing how we differ as players when in a solo campaign. We'd done groups before and that was great and all, but with a solo campaign with your significant other you really get to explore your selfish interests. His druid doesn't much care for dungeon crawling and would much rather explore unique, high-magic environments with occasional plothooks tying his fate to that of a fascist, anti-druid empire; I like low magic/high fantasy city intrigue and a rich cast of NPCs to indulge my bard's troublemaking ways in a society where magic is outlawed. So talk to your girlfriend-- does she want some undead dungeon crawling? Does she want to go and kill some orcs? Does she want to find and befriend a dragon? Become the ruler of some small town? Does she want to start as a first level character, or has her character been adventuring for a while? Does she want a high-magic world? High fantasy? Would she prefer you just run a few pregenerated adventures to ease her in, or would she prefer a world mostly custom-made to her interests?
Feel free to create a few interesting NPCs to help. If you can tie them to her character's life in some way (a sibling, a friend, a lover, whatever she likes) this is when the backstory portion of character creation can really shine. Don't put the focus on them, she should be the one that usually makes the choices and goes in for the epic kill, but having someone to care about/toss her a healing option when you misjudge the CR can really help.
The more you know about what kind of game and what kind of character she wants, and just how immersive she wants the character creation to be, the better you can design an experience that you'll both find enjoyable and hopefully foster a love for the game that you'll be able to share for a lifetime!
. Bonus! And I don't need pics to prove my assertion.
