In a world where the average game sees players tromping through the gore-spattered, phallic-laden, leatherneck power fantasies of every thirteen-year-old boy, the Portal franchise represents more than a series of delightful puzzles layered in a gooey helping of humor so pitch black it'd make Riddick himself squeal with delight. The games serve as a sublime subversion of the first-person shooter genre, and indeed much of the vast, predominately-masculine landscape of gaming as a whole, offering up a feminist interpretation of the conventions of each with a side of cake and potatoes.
Naturally the most prominent examples are the stars of the games themselves, specifically long-runners Chell and GlaDos. It's tempting to redact points from Chell because she's about as blank a slate as one can get, even by Valve's standards. Where the Half-Life games established Gordon Freeman as a remarkably introverted theoretical physicist, he was nonetheless given a place in the world, a backstory, and the eventual role of a postmodern-day, crowbar-wielding Jesus H. Christ. Chell, on the other hand, is punted awake into the testing chambers of Aperture Science with nothing more than a cold greeting and a portal. Any information that would fill in the gaps of her past is either omitted by GLaDos, or else most likely a hammy fabrication. Even taking into account the possibility of GlaDos' wicked barbs being as factual as they are humorous, that still leaves the player to conclude that Chell is a fat, smelly, horrible orphan who doesn't look at all strapping in an orange jumpsuit.
She merits discussion, however, because she's the incredibly-rare female protagonist whose physique or even personality aren't dialed up to eleven just to make a mad grab for sales. She's not the sort of hypersexualized, triple-Z cup, "oh no everyone can see my bum" woman copy-pasted from Lara Croft's nudie pics, and neither is she a slavering, bloodthirsty, borderline-sociopathic murderer who traipses around breaking off her high heels in the men's hairy bums just to prove she as much a man as the boys, such as Wet's Rubi. Her gender is presented as such a non-issue - or, more accurately, not presented in the first place save for the player catching glances of her through adjacent portals - and that's what makes her a compelling case for the game's feminist argument - she's every bit as stoic and capable as Gordon Freeman, and thus at least in their capacities (if not in their roles - survivor vs. messiah) they come across as true equals.
GlaDos is a more obviously feminine figure, characterized through a distinctive woman's voice and an especially catty, cloying personality. Even her design, confirmed by the developer's to have been inspired by Botticelli's Venus, is physically evocative of a woman (http://www.game-ism.com/2008/04/09/glados-followup-shes-your-venus/). The same blogger brings up a fascinating theory, the idea that GlaDos physically resembles a woman hung from the ceiling in bondage, and goes so far as to capture the image through his own rendition of her body as it would appear in human form (http://www.game-ism.com/2008/04/04/still-alive-shes-free/). It's a remarkable bit of artistry, and incredibly suggestive of how restrictive GlaDos' existence truly is. For the entirety of Portal and at least half of Portal 2, GlaDos remains trapped in her chambers; she spends the first halves of both games singularly obsessed with performing tests on Chell, occasionally hinting of cakes and dear and the existence of people besides Chell and herself, but never providing anything concrete, and thus further alluding to how incredibly trapped she is.
Given GlaDos' origins as Catherine, the secretary of Cave Johnson and ostensibly the inaudible voice of reason in Aperture Science, it isn't hard to imagine that her bondage is indicative of her past as a member of what must have been an incredibly patriarchal society. Aperture Science was conceived of well before the feminist movements of the 1970s, after all - though she would have possessed the right of suffrage, Catherine still would have been predominately under the thrall of Cave Johnson, both as a woman and his secretary. Catherine's characterization is almost as slim as Chell's - though she is revealed to possess at least some spark of humor and wit, and even hinted not-so-subtly at sharing a relationship with Johnson, it is worth noting that the majority of the focus of the old Aperture levels is on Johnson himself. It's under Johnson's orders that Catherine is uploaded as an AI into what will eventually become GlaDos, and so in effect GlaDos is born out of Johnson's love for science, and possibly Catherine's love and respect for Johnson. The image of her as a woman in bondage becomes a powerful one in light of its symbolic representation of GlaDos' existence and purpose being determined not by herself, but by a powerful man.
The contrast between GlaDos and Wheatley represents another layer of gender commentary, exploring the primal roles typically associated with men and women. GlaDos, notably, spends her time in the spotlight repairing the testing facilities, polishing them to a shine, and devising entirely new challenges where none had existed before. When Wheatley assumes command of the facilities, however, destruction and chaos soon follow. The turrets are warped into defective models or else Frankensteined together into abominations equal parts horrific and adorable, and the chambers he devises are either blatant copies of GlaDos' chambers, spare rooms he found tucked away, or else a dunderheaded mix of the two, literally crushed together into a barely-functional chamber. GlaDos' climax is (by her standards, at least) something of a subtle, clever affair: the player is gradually carted away from the sterile confines of the final test chamber into an incinerator, which incorporates a clever allusion to baking a cake. Wheatley, on the other hand, literally flings Chell right into a death trap he's been unsubtly telegraphing since the moment the idea occurred to him, and there's absolutely nothing clever, poignant or ironic about it at all. He opts instead for overkill, a gleaming phalanx of, in his own words, "mashy spike plates" that rather neatly capture the average first-person's shooter's attitude towards enemy combatants. Even the means of defeating each serve as stark parallels - GlaDos must be disassembled, the pieces of her mind stripped away until she can't exist anymore, where Wheatley must have personality integrated into his mainframe, in order to overwhelm and corrupt him. In an nutshell, the feminine GlaDos represents an emphasis on creation and mental stimulation, and is thus undone by simplification. The masculine Wheatley represents an emphasis on destruction, imitation, and physical overstimulation, and is quite literally overwhelmed by adding new layers to his own personality.
Finally, there is the players' myriad means of interacting with the world through the Aperture Science Handheld Portal Device, henceforth abbreviated to the portal gun. In and of itself the portal gun represents a remarkable shift in the structure of the game - though physically evocative of the firearms which dominate the landscape of modern gaming, its smooth, even voluptuous shape is indicative of an instrument distinctly feminine in nature. More tellingly, however, is that while referred to as a gun, it remains incapable of a gun's most basic feature - the wounding, destruction, or murder of a target. The portal gun possesses no inherently-violent features, instead used almost exclusively as a means of transportation, whether through the clever application of velocity and momentum, the creation of bridges of light and gravity funnels, or even simply plopping down onto an otherwise inaccessible ledge through a well-placed pair of portals. Though missiles, bombs, and turret fire can be redirected through the portals for combative purposes, this is always only possible in the presence of a force hostile to Chell, and only renders her assailants the architects of their own destruction. Rather than superior muscle or firepower saving her bacon, as is the case in the male-oriented shooters such as Halo or Call of Duty, Chell emerges victorious due to a deft mixture of cunning and tenacity, in a remarkably feminist subversion of the norm.
Sources used: http://www.gamesradar.com/xbox360/f/portal-is-the-most-subversive-game-ever/a-20071207115329881080/g-2006071916221774024/p-3