|Segment||Pinky and the Brain|
|First Episode||The World Can Wait|
|Voice Actor||Tress Macnellie|
Billie is a beautiful female mouse that Brain felt an extreme attraction to. However, Billie found him at first boring and prefered Pinky's company due to their similar taste of humor. However, Pinky's shown to be not interested as he's in love with Phar Fignewton. With Pinky's coaching, Brain attempted to gain her affections by talking similar to Pinky. He got the nickname "Eggy" to please her. But before he could continue to woo her, she was taken by the scientists as a last experiment before turning in for the night.
This resulted in her heightened intelligence, much to Brain's surprise. It would soon be shock as Billie turned out to be smarter than him. As he left, Pinky asked if he didn't love her anymore because she was smarter than him. Brain replied that he did even more but believed they had to continue their plans for world domination more now. When Pinky asked why, Brain answered they had to out of fear she'd beat him to it first. Something that caused even Pinky to gasp in worry.
Billie would make two more appearances in the Pinky and the Brain show. In "Brain Noir" where it was revealed she had a past relationship with Snowball, something he enjoyed rubbing in Brain's face. Brain still had feelings for Billie, despite this. Billie would come to him and appeared she need help but eventually it was discovered that she was working for Snowball to create something.
Snowball was eager to take joy in Brain's betrayal, but Billie would reveal she was the real boss who used them both. She stated she didn't care for Snowball(probably because of his controlling ways) and Brain would never work out because he loved the world more than her. She then reveals what she really had built was a waterpark, just for Pinky. She then had her goons toss him and Snowball over the edge of the waterslide and tried confessing her feelings to Pinky.
She then gave him a passionate kiss, something that shocked Pinky. Whether out of uneasiness or indecesiveness, he jumped into a waterslide. Guessing he rejected her feelings, Billie gave a sigh.
Billie's last appearance was in "You'll Never Eat Food Pellets in This Town, Again!" when Brain dreamed of being an actor. In the dream, Billie's real name was Shelia and married Brain to boost her career as an actress. She quickly kicked him out when he got fired from his show and wanted to move to the country.