Who is Susannah from Legends of the Fall and what is their importance?

Susannah is betrothed and married to Tristan. the couple marries before Samuel's funeral, against Isabel's wishes, as well as her father's wishes. She understands Tristan's need to leave, but she never gets over her grief when he does not return. Her mental health deteriorates under the pressures of being a senator's wife when she marries Alfred. She remains childless, further compounding her mental problems. She manipulates Tristan to try to win him back and finally commits suicide when she fails.