Acorn is a small, sand-coloured spotted tom with golden eyes.
Acorn is born to Feathersong and Maple, a loner who lives by the Yarrowclan camp, along with his siblings, peach, honey and strawberry. He is seen as highly energetic and brave compared to the other siblings, but he often play-fights with his much bigger sister, Peach, while honey and strawberry stay out of it.
At around 5 moons, Peach and Honey are chosen to go to Yarrowclan. Acorn always seemed interested in clan life, so his father told him all about Starclan and their warrior ancestors. About a moon later, Feathersong decides to be Acorn's unofficial mentor, so him and peachpaw can train together.
One day while looking for his father in the yarrow, he gets lost and comes across a large calico, Fox. At first, fox seems nice, even offering to take Acorn home to his mother. Acorn then says that he's waiting for his father, who's the yarrowclan deputy. Fox, hating yarrowclan because of her past there, badly injures the small tom in spite and then runs off and leaves him to die. Feathersong finds his nearly-dead son on the ground, so he instantly picks up his son's limp body and brings it to the yarrowclan medicine cat, Frostbreeze.
Maple finds out about his son, and quickly runs into Yarrowclan camp as fast as she could. Blood was already spilled too much and quickly by the time that Maple arrived, causing him to die and go to starclan.
He is later seen giving the life of bravery to featherstar when he becomes leader of yarrowclan.