Kepler is known for being the more energetic cat out of the two, and he commonly enjoys snuggling and sitting on laps. He likes tampering with household items such as Stephen and Mallory's computer keyboards. His energy can often be seen on the vlog with his interactions of initiating "play time" with Sagan.
As a kitten, Kepler had an extended period of ill health, first noticed by Stephen on Day 1022. His first trip to the vet revealed some stomach issues and a fever. He later developed an abscess, thought to have been caused by playing with Sagan and a fever of 105.4, requiring a second trip to the vet to get the abscess drained and a fresh course of antibiotics. Despite the high cost of the vet trips, Stephen affirmed he was "one of the family". His later recovery manifested itself as a string of behavioral misdemeanors. He later suffered from a bout of ringworm and further stomach issues, destroying a cat condominium by vomiting on it.
Both Kepler and Sagan were intended to be toilet trained as kittens, mainly for the convenience of not having to buy litter and maintain their litter box. Despite early progress, Kepler later started showing erratic toilet behavior. Stephen eventually came to the conclusion the toilet training was causing stress for Kepler, and decided to discontinue training them.
Kepler's behavior and ill health in younger life have become notorious to the point they are referenced even when the cat isn't present. During a game of Telestrations, a drawing for 'Dragon Breath' was guessed instead as 'Kepler vomiting', which Stephen later illustrated, prompting a guess of 'Demon cat puke' which Mallory suggested was "spot on" for Kepler.