Kitty first appears as a Caterpie in As Gastly as Before. He is first seen in a tree and is about to be attacked by a Fire Blast launched by Blaine's Growlithe, which was attacking Agatha's Gastly. Yellow immediately jumps into action and sent Pika to rescue him by means of her fishing rod. After this, Yellow leaves with Blue to go train, with Caterpie watching her go.
In Can't Catch Caterpie!, Caterpie is revealed to have followed Yellow all the way from Celadon City. Blue tells Yellow to capture him and train him to a higher level. Yellow then sends out Ratty to try to catch him. The two spent all day trying but could not find a way to do it without hurting him. Blue eventually steps in and helps the two capture Caterpie, which she nicknames Kitty.
In the next chapter, Yellow leaves Kitty with Pika as she goes off to run an errand. After Yellow races off Kitty spots a leaf floating past, and chases after it. The leaf floats over to the edge of the cliff, so Pika chases after Kitty to stop him from falling off the edge. Kitty then chases the leaf and almost runs into a cactus. Pika runs in front of Kitty and blows the leaf away, with then flies off in a gust of wind. Pika finds another leaf and gives it to Kitty when suddenly a herd of Pidgeotto fly past and pick Kitty up in the process. Pika climbs the cliff, and then jumps off and shocks the Pidgeotto. The Pidgeotto drops Kitty, and Pika catches him, with Kitty looking very distressed. They land next to a tree and some flowers, and Kitty eats all the flowers, recovering his health and cheerfulness. The two then promptly fall asleep.
In Eradicate Raticate!, Kitty is sent out alongside his teammates to battle against Lance and his Dragonite. Kitty attacks with String Shot, but Dragonite simply shrugs it off and throws Kitty away. He is later used in a team attack to try and stop Lance from shooting bubbles. Yellow had Kitty cover the volcano crater in String Shot to form a net that is doused by her Omny's water, linked to Ratty's sensitive whiskers. Upon Lance triggering the net, Ratty signals to Pika to send electricity down the net, forcing Lance's bubble into view. Yellow then orders Dody to break the bubble with Drill Peck, but the attack proves too weak.
As a Caterpie, Kitty was shown to be very ditsy and easily distracted. He would often wander around and get himself into dangerous situations, usually resulting in Pika getting hurt but Kitty walking away unharmed. Kitty has also been shown to be extremely affectionate of Yellow, shown when he followed her all the way from Celadon City just to stay with her. Kitty can frequently be seen rubbing affectionately against Yellow or wagging his tail happily.
Yellow often uses Kitty's String Shot in a variety of ways to help her out. Some examples include being used as a net, a means of tracking, a line for her fishing pole, and as a cast.