Berlioz is usually very quiet, but mischievous. He enjoys playing the piano, as seen when Duchess asks him to practice his Scales and Arpeggios with his sister Marie. He isn't easy to impress and can be annoyed rather quickly, usually he's the first to make a snide look or comment toward things he doesn't like. Still, he's shown to be just as naive and sweet-natured as his siblings.
Berlioz has blue eyes, a small and skinny black-furred body with a lighter gray stomach. He also wears a small red ribbon around his neck that is tied. Being a kitten, he's fairly small.
Berlioz first appears in The Aristocats during the opening credits, which display some of the scenes that he will appear in later.
Berlioz is next seen during the carriage ride during the beginning of the movie, where he rides on Frou-Frou's head. At the end of the carriage ride, his mother, Duchess, reminds him to thank Frou-Frou for letting him ride on her head (which he does). After asking how his thank you was, Duchess comments, "That was very nice."
Later, Berlioz is in a race with his siblings to see who goes through the front door first. When Marie states that she has the right of way, Berlioz sides with Toulouse's argument that Marie isn't a lady, even going as far to say that she's "nothing but a sister." Soon, Berlioz ends up in a fight with his sister (with Toulouse encouraging his brother from the sidelines). Berlioz tickle tortures Marie but ends up receiving punishment when his sister refuses to fight fair by tugging at his bow tie. Eventually, the fight ends when Toulouse accidentally knocks down a candle and it lands on Marie’s head. Duchess comes in to break up the fighting, insisting that fighting won't make them sophisticated (stating later that "Aristocrats do not practice biting and clawing and things like that"). Berlioz insists that Marie started it, while Marie retaliates, "Ladies do not start fights, but they can finish them!"
Soon enough, it's time for the family to concern themselves with self-improvement. While Toulouse goes to practice painting, Berlioz and Marie go to the piano to practice their music. Though there is a brief tiff between Berlioz and Marie when (thanks to Berlioz's piano-playing) Marie gets her tail squished, the fight ends and "Scales and Arpeggios" begins. Berlioz gets a little show-offish and, after Toulouse decides to join in on the playing, Berlioz and he end up in a piano battle that ends in a draw.