In Summer's younger days, she enrolled to Beacon Academy and became a member and leader of Team STRQ, along with Taiyang Xiao Long, Raven Branwen and Qrow Branwen. She and her team eventually graduated to become Huntsmen and Huntresses.
At some point later in life, Summer entered a relationship with Taiyang after Raven had left him, the two having a daughter, Ruby Rose, and Summer took to raising Raven's daughter, Yang Xiao Long, as well. Summer's family lived in Patch as she left regularly to carry out missions. According to Yang, Summer was like a "supermom", "baker of cookies" at home and "slayer of giant monsters" due to her Huntress career.
It is revealed by Qrow that Summer acknowledged and utilized her abilities through her Silver Eyes. It is unknown when and how she discovered her abilities and to what extent.
At some point, Summer encountered Salem, where she told her that she'd be stopped without having to be killed. This encounter was memorable enough for Salem to remember her words.
When Yang and Ruby were still young, Summer went on a mission and never came back. Specific details surrounding her fate are undisclosed or unknown. According to Qrow, the mission in question was Summer's secret and even Ozpin seemed in the dark about her disappearance. According to Yang, Ruby was likely too young to be able to fully comprehend what had happened.
Taiyang did not handle the news nearly as well as Ruby; Yang saying that he simply "shut down". It is after this that Yang learned Summer was the second love that he had lost, the first being Yang's mother, which makes Summer Yang's stepmother.
It is implied in "Gravity" that Salem is responsible for Summer's death.
In "The More the Merrier" Ruby attempts to sway Raven into joining the fight against Salem. Raven responds, somewhat contemptuously, "You sound just like your mother", before beginning the attack on Haven.
In "Pomp and Circumstance", Ruby and Qrow have a conversation about Summer's last mission and the unknown nature of her disappearance. Ruby also wonders what she would've done had she learned the truth about Salem, to which Qrow tells her he believes she would've pressed on, like her.
During "Gravity", Ruby tells Salem that even if killing her is impossible, she can still be stopped. However, Salem looks at the girl and tells her that's exactly what Summer said, and that in the end it wasn't true. Ruby proceeds to break down in grief, thinking of her mother.