Louie was really more David Michael's dog than anyone else's. Louie slept with David Michael every night. It was also David Michael who discovered the rock song called Brother Louie that Louie barked along with every time they played it.
After Louie died, Kristy began wearing a baseball cap with a picture of a collie on it in Louie's memory.
Louie had arthritis and poor eyesight when he was older. He was unable to walk on stairs and lost control of his hind legs. He was eventually put to sleep at the vets because the Thomases and the Brewers didn't want him to suffer any more. Louie dislikes going to the vet, even for a check up.
His toys were buried in the Brewer-Thomas' backyard, since they couldn't bury Louie.
Louie was made fun of by the snobs (Tiffany and Shannon Kilbourne) in Book #11 Kristy and the Snobs. They were repenting when Louie died, however, and attended his funeral when invited by Karen. Also, Shannon gave Kristy one of her dog's (Astrid's) puppies later on.