The name of Linus and Lucy's younger brother is Rerun Van Pelt.
Rerun was introduced by Charles M. Schulz in the Peanuts comic strip in 1972, with his first full appearance following in 1973. Lucy had wanted a baby sister, but when another brother arrived, she complained that it was a “rerun” of Linus. The nickname stuck, and it became the character’s familiar name.
Rerun is noticeably younger than Linus and Lucy and was initially shown as a toddler. For much of the 1980s, he appeared riding in a child seat on the back of his mother’s bicycle, often commenting on the hazards of the journey. Schulz later used him more frequently because the character allowed Peanuts to explore preschool and kindergarten experiences.
A common mix-up is confusing Rerun with Linus, since the two brothers look very similar. Rerun also became especially prominent in the late comic-strip years and starred in the 2003 animated special I Want a Dog for Christmas, Charlie Brown.