Personality, what can you do with it, and what it tells us about happiness in humans and other animals?Diversity is a hallmark of all living systems, and understanding consistent behavioral differences is essential to our understanding of human and animal psychology. In the talk, I will present several recent works showing how we can quantify personality in animals and how it helps us understand them and ourselves better. In the lab, we use machine-learning tools to track the behavior of multiple species, including mice, feral cats, cows, and humans. We use these measurements not only to determine their personality but also to infer their emotional state. In a recent study, we have started using this cross-species personality data to help us establish an automatic universal translator between animals and humans. Lastly, we show how individual differences allow animals to use cryptographic principles that help them survive in nature.