Millie was a kittypet who lived near Graystripe when he was captured and made a kittypet. Millie and Graystripe first met each other after Graystripe was beaten by Duke, an agressive kittypet. They almost instantly became friends. Graystripe told her much about the forest he lived in and Clan life. Millie, unlike most kittypets, become very interested in the ways of Clans. To cheer her new friend up she showed him a small woods near the Twolegplace.
Millie was taught how to hunt and fight under Graystripe's teachership. They even beat Duke and his band of cats. Millie also persuaded Graystripe go and find his clan that he got separated from. She loved him enough to go with him.
Millie became a warrior when they found ThunderClan. She fought against ShadowClan in the border battle, and told Firestar the medicine cat kept her from getting sick. Millie also refused to get a warrior name, saying she want to remain Millie.
After they joined ThunderClan, she and Graystripe had three kits, named Blossomkit, Briarkit and Bumblekit. She hides in Firestar's den when the four Clans fight in Eclipse. She and Briarkit catch greencough, but Silverstream allows them to recover, sparing Graystripe from getting another broken heart. Millie is slightly overprotective of her kits.
Millie and Graystripe are proud of their kits, now apprentices. When Briarpaw's legs are paralized by the tree, she becomes way overprotective of her. When Jayfeather tells her Briarpaw (now Briarlight) is going to be paralyzed for life, she often neglects Bumblestripe and Blossomfall to care for Briarlight, which leads to Blossomfall joining the Dark Forest.