A few thoughts on the narrative of Portal 2 and a look in depth of what I consider the 5 characters of the series.
The first game's story was simple. GLaDos uses Chell to perform experiements with the Aperture Science Portal Device. Eventually GLaDos decides her experiments are finished, and attempts to kill Chell. Chell, now having what I would consider advance Portal training, easily escapes and "destroys" GLaDos.
There is very little story here to analyze and tear apart. You can see the ratman den's. You can assume he is around and even helping Chell. Basically this was meant to be pure gameplay, icing on the Orange Box cake. But it gained popularity, so they had to deepen the story. This brings us to the comic.
The story that bridges the two games is Valve's comic, Labrat. The story follows Ratmen, a schizophrenic scientist during GLaDo's installation of the conciousness core, Chell's escape, and her eventual recapture. In it you see that Chell is dragged back into the lab by some kind of emotion core. Ratmen follows her back into the Facility to find they put her into the relaxation chambers, which have been deactivated. He risks his life to save her, but guarentees she is next up for experiments. The comic ends with him entering a sleep chamber similar to the one Chell emerges from at the beginning of Portal.
This is a really good way to get people interested in teh story. The only real signs of humanity in the first game were the ratman nests. The webcomic shows us a man going off his meds, muttering to his companion cube just to survive. He is always against GLaDos, even when the lab was in human operation. A fun allusion throughout the comic is Schrodinger's cat. Even the ending points out that Chell in her relaxation chamber is neither alive nor dead.
The second game's plot is the grandest so far in story. Chell awakes to Wheatley, a core tied to a railing, wanting her to obtain the portal gun. She fetches it and Wheatley removes himself from teh tracks and requests that Chell carries him to "Her" room. Of course GLaDos's chamber is still in ruin from the end of game 1. Chell digs through the rubble, trying to find the Escape Hatch fuse to activite it and get launched to freedom. Wheatley is set into a core control system, trying to find the fuse when the platform rises up flipping switches and controls which awaken GLaDos. She quickly dispatches Wheatley and sends Chell back to testing while she rebuilds the facility. Just as GLaDos is finished with construction, Wheatley saes Chell and they go about deactivating her many deadly turrets and neurotoxin. After they finish their sabotage session, they head to GLaDos. After they leave GlaDos weakened, Chell is given the opportunity to replace GLaDos with Wheatley. she does so without question, giving Wheatley full control of the Facility. Wheatley almost instantly goes mad with power, putting GLaDos in a potato and sending her and Chell into the deep bowels of ancient Aperture. Chell and GLaDos fight their way back to Wheatley and send him to the moon. With GLaDos back in power, she finally decides what to do with Chell. GLaDos releases her.
There is a lot to say about this story, and most of it has been said. Plenty of people have touched the Greek references and the possibility of Carolyn being Chell's mother. I haven't seen others touch what I saw was the main story. The first time I played through the game, I saw it as Wheatley's tale. He is a nervous peon who rises to power, goes mad with it, and fall from grace. This story is common in media, be it Tony Montana's empire in Scarface or King John in the legend of Robin Hood. Chell (the player) is simply a tool Wheatley uses to obtain his goal. I can easily imagine the story without Chell. GLaDos could be changed to an extent and the plot remains untouched. Cave Johnson and the ratman are both there for one or two plot points, but are mostly there for comic relief. But I feel wheatley is the focal character, someone the story focuses around.
The characters are a bit harder to approach. Chell, as most have pointed out, is a Tabula Rasa, a blank slate. She is meant to be mute and characterless so the player may project themselves upon her.
Ratmen is a complex character we don't get to see much of. We know he is Schizophrenic and off his meds. We know he is brilliant. We also know he tries to avoid direct conflict. I personally hope through either more comics or DLC they expand his character.
Cave Johnson is nothing more than a series of recordings, but I feel as though he is vital to the plot and build of the game. He is shown to be a man's man and having possibly no knowledge as to the definition of science is. Cave simply has the money to "Throw science at the wall and see what sticks." He is willing to sacrifice human life to make a simple profit. He is shown to use his own workers as lab rats just to see what would happen if you combine human DNA with that of a Mantis's. Out of all these almost psychotic qualities, a few noble aspects shine through. He is kind and even somewhat loving to Carolyn, his assistant and assumed lover.
GLaDos has gone through a lot in these games. She awakens Chell simply for some experiements. In portal 1, she is almost emotionless until the finale. She simply states backhanded compliments and the occasional threat. The ending has her explode with a homicidal nature and insane rambling that would make a slasher film villain seem normal. The second game shows her with more of an emotional range. I always assumed the destruction from the first game corrupted some of her files, forcing her to use older, less efficient systems. This unlocks Carolyn within her motherboard, which gives GLaDos some semblance of emotion. The only true redeeming moment GLaDos gets is the Finale, where she saves Chell from dying in the vacuum of space.
Finally we have Wheatley. He is a core designed to have bad ideas. There is plenty to point this out even before he creates TurretCubes. He reactivates GLaDos when trying to find the Escape Hatch. He talks bad on Humans when he is stuck depending on a human. He frequently gets lost in a facility he has spent his entire life in. But his character is more than an electronic buffoon. Wheatley shows fear of death, but a willing to risk his own life to escape. Wheatley lies frequently, either for his own ends, or simply to try and impress others. By the end, however, he is apologetic and honest.
The story has wrapped itself up rather nicely. The story was entertaining and the conclusion is definitive. But if there is one thing media has shown us, if there is a profit in it, they will make a sequel.