Here I have tried to include brief descriptions of all the major recurring characters on Six Feet Under along with a listing of the episodes they appear in. Each actor's name is linked to his or her IMDb profile in case you'd like to know more about them. :) This is very much a work in progress!

Vanessa Diaz (Justina Machado)
Wife of Federico Diaz. Vanessa works as a nurse and is the mother of Rico's two children, Julio and Augusto. After the death of her mother, she becomes depressed and starts using various antidepressants. She and Rico split up for a while after he begins spending too much time with another woman, but they eventually get back together and Vanessa helps Rico set up his own funeral home.
Episodes: 39 (1.2 - The Will, 1.8 - Crossroads, 1.11 - The Trip, 1.12 - A Private Life, 1.13 - Knock, Knock, 2.2 - Out, Out, Brief Candle, 2.6 - In Place of Anger, 2.7 - Back to the Garden, 2.8 - It's the Most Wonderful Time of the Year, 2.13 - The Last Time, 3.7 - Timing and Space, 3.8 - Tears, Bones and Desire, 3.9 - The Opening, 3.10 - Everyone Leaves, 3.11 - Death Works Overtime, 3.12 - Twilight, 3.13 - I'm Sorry, I'm Lost, 4.1 - Falling Into Place, 4.2 - In Case of Rapture, 4.3 - Parallel Play, 4.4 - Can I Come Up Now?, 4.6 - Terror Starts at Home, 4.7 - The Dare, 4.8 - Coming and Going, 4.10 - The Black Forest, 4.11 - Bomb Shelter, 4.12 - Untitled, 5.1 - A Coat of White Primer, 5.2 - Dancing For Me, 5.3 - Hold My Hand, 5.4 - Time Flies, 5.5 - Eat a Peach, 5.6 - Rainbow of Her Reasons, 5.7 - The Silence, 5.8 - Singing for Our Lives, 5.9 - Ecotone, 5.10 - All Alone, 5.11 - Static, 5.12 - Everyone's Waiting)

Billy Chenowith (Jeremy Sisto)
Younger brother of Brenda Chenowith. Billy is a photographer who suffers from pibolar disorder. He is also bizarrely obsessed with his sister Brenda to the point that he attacked her with a knife at one point and has made sexual advances towards her more than once. He spends some time in a mental institution early in the series. He also gets involved in a serious relationship with Claire Fisher, but she breaks up with him after he goes off his medication because it frustrates his creativity.
Episodes: 30 (1.1 - Pilot, 1.5 - An Open Book, 1.6 - The Room, 1.7 - Brotherhood, 1.8 - Crossroads, 1.9 - Life's Too Short, 1.10 - The New Person, 1.11 - The Trip, 1.12 - A Private Life, 1.13 - Knock, Knock, 2.8 - It's the Most Wonderful Time of the Year, 2.9 - Someone Else's Eyes, 2.12 - I'll Take You, 3.7 - Timing and Space, 3.9 - The Opening, 3.10 - Everyone Leaves, 3.11 - Death Works Overtime, 4.9 - Grinding the Corn, 4.10 - The Black Forest, 4.11 - Bomb Shelter, 4.12 - Untitled, 5.1 - A Coat of White Primer, 5.2 - Dancing For Me, 5.3 - Hold My Hand, 5.4 - Time Flies, 5.5 - Eat a Peach, 5.6 - Rainbow of Her Reasons, 5.10 - All Alone, 5.11 - Static, 5.12 - Everyone's Waiting)

George Sibley (James Cromwell)
George first appears as a guest at a funeral hosted by Fisher and Diaz. He meets Ruth and comforts her over the disappearance of her daughter in law, Lisa. The two embark on a whirlwind courtship and get married within a matter of weeks. It turns out that George has six previous wives he neglected to tell Ruth about as well as some psychological issues resulting from his mother's suicide. He suffers a nervous breakdown and it becomes too much for Ruth to take care of him, so they divorce, but they eventually get back together.
Episodes: 27 (3.11 - Death Works Overtime, 3.12 - Twilight, 3.13 - I'm Sorry, I'm Lost, 4.1 - Falling Into Place, 4.2 - In Case of Rapture, 4.3 - Parallel Play, 4.4 - Can I Come Up Now?, 4.5 - That's My Dog, 4.6 - Terror Starts at Home, 4.7 - The Dare, 4.8 - Coming and Going, 4.9 - Grinding the Corn, 4.10 - The Black Forest, 4.11 - Bomb Shelter, 4.12 - Untitled, 5.1 - A Coat of White Primer, 5.2 - Dancing For Me, 5.3 - Hold My Hand, 5.4 - Time Flies, 5.5 - Eat a Peach, 5.6 - Rainbow of Her Reasons, 5.7 - The Silence, 5.8 - Singing for Our Lives, 5.9 - Ecotone, 5.10 - All Alone, 5.11 - Static, 5.12 - Everyone's Waiting)

Russell Corwin (Ben Foster)
Russell is one of Claire's classmates at art school. She assume's he's gay and the two become friends, but when he tells her he's straight, their relationship develops into a romance. Later, Claire finds out that Russell slept with Olivier, their teacher, and breaks up with him. She then finds out that she's pregnant with Russell's baby and has an abortion without telling him. Russell is furious when he finds out. Then Russell gets even angrier at Claire when she takes all the credit for an art project he feels he was also partly responsible for. They fight and don't see each other for some time. Later when they meet again Russell is chasing after another classmate, Jimmy.
Episodes: 23 (3.1 - Perfect Circles, 3.2 - You Never Know, 3.3 - The Eye Inside, 3.4 - Nobody Sleeps, 3.5 - The Trap, 3.6 - Making Love Work, 3.7 - Timing and Space, 3.8 - Tears, Bones and Desire, 3.9 - The Opening, 3.10 - Everyone Leaves, 3.11 - Death Works Overtime, 3.12 - Twilight, 4.1 - Falling Into Place, 4.3 - Parallel Play, 4.4 - Can I Come Up Now?, 4.5 - That's My Dog, 4.6 - Terror Starts at Home, 4.7 - The Dare, 4.9 - Grinding the Corn, 4.10 - The Black Forest, 4.11 - Bomb Shelter, 4.12 - Untitled, 5.8 - Singing for Our Lives)

Lisa Kimmel Fisher (Lili Taylor)
An old friend and roommate of Nate's from Seattle. When Nate returns to Seattle for a visit, he and Lisa sleep together and she becomes pregnant with his child. She moves to LA when she gets a job working as a chef for a movie producer. Initially she wants Nate to sign away his rights to her baby but after Nate and Brenda split up, he and Lisa decide to get married and raise their daughter Maya together. Their marriage is very unhappy because Lisa worries that Nate still loves Brenda. Lisa goes on a trip to visit her sister one day and never returns. She is missing for some time until her body washes up on shore and it is assumed that she committed suicide, but later it is revealed, or at least strongly suggested, that her brother-in-law, whom she was having an affair with, killed her.
Episodes: 23 (2.4 - Driving Mr. Mossback, 2.9 - Someone Else's Eyes, 2.10 - The Secret, 2.12 - I'll Take You, 2.13 - The Last Time, 3.1 - Perfect Circles, 3.2 - You Never Know, 3.3 - The Eye Inside, 3.4 - Nobody Sleeps, 3.5 - The Trap, 3.6 - Making Love Work, 3.7 - Timing and Space, 3.8 - Tears, Bones and Desire, 3.9 - The Opening, 3.10 - Everyone Leaves, 3.11 - Death Works Overtime, 3.12 - Twilight, 3.13 - I'm Sorry, I'm Lost, 4.1 - Falling Into Place, 4.7 - The Dare, 4.9 - Grinding the Corn, 4.12 - Untitled, 5.1 - A Coat of White Primer)

Margaret Chenowith (Joanna Cassidy)
Mother of Brenda and Billy. Margaret is a psychologist. She met her future husband Bernard when she was an intern. The couple let their daughter Brenda be the subject of Dr. Gareth Feinberg's research, which would become a cause of a lot of tension within the family. Bernard and Margaret had an open marriage, but they split up briefly when Bernard failed to follow the couple's rules regarding infidelity. They soon renewed their vows, but Bernard died of cancer not long after. Soon after her husband's death, Margaret became lovers with Olivier Castro-Staal.
Episodes: 21 (1.5 - An Open Book, 1.10 - The New Person, 2.4 - Driving Mr. Mossback, 2.7 - Back to the Garden, 2.8 - It's the Most Wonderful Time of the Year, 2.10 - The Secret, 2.11 - The Liar and the Whore, 2.12 - I'll Take You, 3.7 - Timing and Space, 3.9 - The Opening, 3.11 - Death Works Overtime, 4.4 - Can I Come Up Now?, 4.5 - That's My Dog, 4.11 - Bomb Shelter, 4.12 - Untitled, 5.1 - A Coat of White Primer, 5.2 - Dancing For Me, 5.5 - Eat a Peach, 5.7 - The Silence, 5.10 - All Alone, 5.12 - Everyone's Waiting)

Nathaniel Fisher (Richard Jenkins)
Husband of Ruth Fisher, father of Nate, David, and Claire. Nathaniel married Ruth after she became pregnant with their first son. He opened his own funeral home, which he named Fisher and Sons, where he worked as a funeral director and mortician until his death in the first episode of Six Feet Under when the hearse he was driving to pick his son Nate up at the airport was hit by a bus. Nathaniel is a rather mysterious figure: he had a private room that no one else knew about and his wife and children realize after his death that none of them knew him very well. He often appears to them in visions.
Episodes: 19 (1.1 - Pilot, 1.2 - The Will, 1.3 - The Foot, 1.6 - The Room, 1.9 - Life's Too Short, 1.13 - Knock, Knock, 2.1 - In the Game, 2.3 - The Plan, 2.8 - It's the Most Wonderful Time of the Year, 2.12 - I'll Take You, 3.1 - Perfect Circles, 3.13 - I'm Sorry, I'm Lost, 4.2 - In Case of Rapture, 4.9 - Grinding the Corn, 4.12 - Untitled, 5.4 - Time Flies, 5.9 - Ecotone, 5.10 - All Alone, 5.12 - Everyone's Waiting)