Sally Jackson is Percy's mom. She works at a candy shop in Manhattan and lives in an apartment in Queens, NY with her smelly, mean husband, Gabe. She has a love of life, and she loves her son more than anything else in the world. She doesn't coddle or spoil Percy, though. Percy imagines what it would be like if he were to run away from his boarding school and go home:
She'd hug me and be glad to see me, but she'd be disappointed, too. She'd send me right back to Yancy, remind me that I had to try harder, even if this was my sixth school in six years and I was probably going to be kicked out again. I wouldn't be able to stand that sad look she'd give me. (1.79)
Sally met Poseidon on Montauk Beach one summer and fell in love. She married Gabe later on primarily because Gabe is smelly, and his stench would protect Percy from monsters. You see, Sally has known all along what being a demi-god will mean for Percy. She is absolutely aware of what the immortal world is like. She tries to protect Percy for as long as possible, but realizes that she must let him go for good to train with other demi-gods.
Percy tells us about his mom's life:
Her name is Sally Jackson and she's the best person in the world, which just proves my theory that the best people have the rottenest luck. Her own parents died in a plane crash when she was five, and she was raised by an uncle who didn't care much about her. She wanted to be a novelist, so she spent high school working to save enough money for a college with a good creative-writing program. Then her uncle got cancer, and she had to quit school her senior year to take care of him. After he died she was left with no money, no family, and no diploma. (3.6)
When Sally met Poseidon, we can imagine that they must have had a pretty wonderful connection. But what would dating a god really be like? It certainly seems like Sally is still in love with Poseidon. Even after marrying Gabe, she has kept her maiden name, "Jackson."
Sally has "a rebellious streak" (3.89) and likes to seek out blue food because Smelly Gabe told her once that blue food doesn't exist. So, she makes a point of finding things like blue corn chips and blue candy. She doesn't like to believe that something is not possible when others tell her that it is not possible. She is an optimist, a go-getter.
At the end of the novel, when Percy asks his mom why she won't go and live with Poseidon in his kingdom in the ocean, she gives him a very interesting answer, demonstrating her fierce sense of independence:
"I think you're enough like me to understand. If my life is going to mean anything, I have to live it myself. I can't let a god take care of me…or my son. I have to…find the courage on my own. Your quest has reminded me of that." (21.171)
At the end of the day, it's pretty clear that Sally Jackson is totally rad. She, like Grover, has a huge heart, and she's not afraid to use it.