Here's Jacob in his awkward teenage years. He was trying to look scary to fit in with the asuras who were a rather violent and scary bunch, so he used his shape shifting abilities to try and look as demonic as he could. And while they had a pretty bad influence on him, eventually turning him into a demon, his yaksha nature was still more interested in fertility and love, and he eventually left the asuras and toned his appearance down. Looking back he just thinks he look like a complete dork.
Don't worry Jacob, it's perfectly normal for teenagers. We all go through a period where we try to look cool or scary but in hindsight just looked incredibly stupid because we had no idea what we were doing.
Nini haven't changed much though. She was just a simple country kappa who just wanted to eat babies and grab butts. She looked the same only with smaller breasts and not as "well fed". ;)