Nami

One of the twin sisters who are patients at the clinic. The two of them are nearly identical, except for the color of their tails. (Nami's is brown with a white tip.)
It's not just their looks, the twins' personalities are identical as well, and they often speak in synch or finish each other's sentences. Maybe they're telepathic?
The twins are innocent, energetic, and mischievous Calico kittens. The two of them are the clinic's "mascots".
Their condition is an accelerated heart rate. Though unaware of their heat cycle, they do illicit things with each other. This doesn't improve their condition, however.