She is the third of the Executioner's victims, representing the sin of envy - where cinder blocks where chained to her feet and she was thrown into a lake, drowning her.
In 1968, a young Brenda was unable to attend the prom because her friend with benefits, Ronald Edwards, got her pregnant. Wanting to kill Sonja (Ronald's girlfriend at the time) for being cruel and vindictive towards her, she waited for her car (full of all her other friends too) to drive under the bridge she was standing on - she planned to drop a large cinder block off the edge, hitting Sonja on the way down.
After they came, Brenda dropped the block only for it to accidentally hit Brenda's best friend Ada, who wasn't supposed to be with them. The injury caused her to fall into a vegetative state, into which she is no longer aware of anything anymore.
Coming back to Waterbury after many years, Brenda is welcomed by Sonja, who has now married Ronald.