Robert was killed at the line of duty when Elle was only a little girl. She was devastated because she had been very close with her father, as well as the fact that she yelled at him for missing her first bicycle ride prior to his death. The latter haunted Elle for many years since it was the last thing she said to him. It is revealed that he called Elle "Peanut".
On Criminal Minds
After being shot by Randall Garner and sent to the emergency room, Elle sees her father in a hallucination and has a heart-to-heart talk with him, who makes her realize that she has to return and fight for her life. After not joining the team as they investigate the case of a serial killer targeting children, Elle visits her father's grave and tells that she was sorry before being confronted by Hotch, who followed her to the cemetery. It is at this moment Elle resigns from the BAU.