How faithful are the characters to the real people they portrayed?

There are no composite characters in the film. However, 6 people were cut from the film entirely. These are: Juan Menendez, Julio Martinez-Lamas, Felipe Maquirriain, Carlos Valeta, Guido Magri and Daniel Shaw. Menendez died in the avalanche, Martinez-Lamas and Maquirriain both died of injuries on the first night, Magri was among those sucked out the back of the plane in mid-air, and Valeta and Shaw both fell out the back of the plane as it was sliding down the mountain. Shaw died instantly; Valeta survived and was seen stumbling down the mountain towards the plane shortly after the crash, but sank into the snow and out of sight. Those who tried to walk out to him had to turn back because the snow was getting too deep.

The rest of the people on the plane all have a specific counterpart in the film. The names of the survivors were kept the same, as were the names of Javier Methol's wife Liliana, and Nando's mother Eugenia and sister Susana. The names of the other people who died were changed for legal reasons. They are as follows (names of their film counterparts are in brackets):

Survivors:
Nando Parrado
Roberto Canessa
Adolfo "Fito" Strauch
Eduardo Strauch
Daniel Fernandez
Carlitos Paez
Roberto "Bobby" Francois
Roy Harley
Ramon "Moncho" Sabella
Gustavo Zerbino
Antonio "Tintin" Vizintin
Javier Methol
Jose-Luis "Coche" Inciarte
Pedro Algorta
Pancho Delgado
Alvaro Mangino

Victims:
Liliana Methol
Eugenia Parrado
Susana Parrado
Marcelo Perez (Antonio Balbi)
Gustavo Nicolich (Victor Bolarich)
Enrique Platero (Pablo Montero)
Daniel Maspons (Juan Martino)
Diego Storm (Hugo Diaz)
Numa Turcatti (Rafael Cano)
Rafael Echavarren (Federico Arranda)
Arturo Noguiera (Alberto Antuna)
Carlos Roque (Fraga, the mechanic)
Graciela Mariani (Mrs. Alfonsin)
Francisco "Panchito" Abel (Felipe Restano)
Julio Ferradas (the pilot)
Dante Lagurara (the copilot)
Ovidio Ramirez (the steward)
Ramon Martinez (the navigator)
Fernando Vasquez (Alex Morales)
Gaston Costemalle (Tomas Alonso)
Alexis "Alejo" Hounie (Jorge Armas)
Dr. Francisco Nicola (Dr. Solana)
Esther Nicola (Mrs. Solana)

As for how accurate the portrayal of the characters are, that varies. Some people had their roles reduced so that the film could have main characters. For example, Eduardo Strauch, the guy in the film who is depressed and given a red shoe by Nando to cheer him up, was much more assertive in real life and seems to have taken a quasi-leadership role over the group after the avalanche. The "we're going to get out of here on our own" speech that Nando gives in the film after their search is called off was actually given by Gustavo Nicolich in real life. Pedro Algorta, who had a fairly big role in real life in helping to organize the group and in convincing those who were reluctant to cannibalize to join in, is reduced to a background extra in the film.

As far as their characterization, its hit and miss. Nando Parrado and Roberto Canessa seem accurate enough, as do Carlitos Paez, Javier and Liliana Methol, Antonio/Marcelo, etc. Some personalities seem deliberately off though. For a few examples, in real life Tintin was described as a bullyish, trouble-making person who didn't get along with others and wasn't well liked. In the film, he's a quiet, friendly guy. Rafael and Arturo (renamed Federico and Alberto in the movie) did not get along at all in real life, but in the film they become the best of friends. I can find no evidence that Roy was close to the team captain like they show in the film. His closest friends in real life were Carlitos Paez, Roberto Francois, Diego Storm and Gustavo Nicolich. The film makes him out to be the team captain's lapdog. Probably most notably, there is no source whatsoever that the plane's mechanic, Carlos Roque, acted crazy like he does in the film. Obviously this was thrown in for comedic relief, but I always thought it was very disrespectful to the man and his family.