"Please ... after everything I've done for you, after everything we've done for each other, at least tell me why."- Marlon West.
In The Unusual Suspect, we are introduced to Marlon West during his trial for murder. His sister, Hannah (a child prodigy) takes the stand and confesses to the murder he was standing trial for committing, even wearing a bloody shirt.
When the siblings are re-introduced in season eight, Sara Sidle admits to having kept tabs on Hannah after she and her brother both were exonerated from their murder charge.
We learn that both of their parents were killed in a car accident about a year prior to Goodbye and Good Luck, that Hannah graduated from Harvard and now teaches at the college Marlon attended.
The two became even closer after their parents' deaths, but Hannah became clingy and ultimately was the cause of Marlon's failed relationship with Kira Dillinger .
The Unusual Suspect
Marlon was on trial for the murder of one of his female classmates, Stacey. During the trial, his 12-year-old sister, Hannah takes the stand and confesses that she herself murdered Stacey, not Marlon.
The judge grants the prosecution 72 hours to re-examine the evidence before the jury delivers its verdict.
In the end, Marlon is found not guilty by the jury. Hannah is set to be prosecuted for Stacy's murder, but, before she is imprisoned, she asked to talk to Sara alone, where she admits that she did not kill Stacy, and was, in fact, covering for Marlon. 
Goodbye and Good Luck
A broken tooth from the crime scene ties Marlon to the death of Kira Dillinger, since it appears that Kira punched him and embedded a tooth in her hand. Kira, however, had slept with another man that night, and both were high on GHB.
It turns out that their sexual lubricant was spiked and Marlon's fingerprint was on the tube.
All the evidence points to Marlon being the murderer, but Sara is convinced that Hannah is the guilty one.
She believes Hannah wanted to get rid of Kira to have her brother all to herself. Marlon confesses to Sara that he had Hannah help him spike the lubricant, but he wouldn't have physically harmed Kira. Sara believes him.
They try to get Hannah to confess to Marlon privately, but she notices immediately that he's wearing a wire and avoids answering Marlon's questions directly. When asked why she killed Kira her response was, "because I love you."
Not long after Hannah's visit to her brother's cell, Marlon hangs himself in his jail cell.
When Sara arrives at the college campus to bring Hannah the news, Hannah believes that Sara is lying, trying to manipulate Hannah into a confession.
When Sara shows Hannah a picture of her deceased brother, Hannah is devastated and breaks down. It is not known whether Hannah was ever charged with Kira's murder.
- Stacy Vollmer (killed by Marlon with Hannah's help; forced out of a shower with a sodium trap, then slipped and fell in a flight of stairs)
- Kira Dillinger (killed by Hannah alone; drugged with GHB and pushed out a window)