Nepeto Leijon seems to be a gender swapped form of Nepeta Leijon. He has mostly the same personality but can get a bit rude at times and his emotions are usually more extreme but he tries to keep them under control. He is terrified by female Gamzees, and is wary around regular Gamzees.
He had a normal life. He had an unrequited crush on Karkit Vantas, who happened to be with Torezi Pyrope at that time. But their relationship didn't work out and Karkit saw that Nepeto understood her better and they decided to become Matesprits.
Eventually Gamzie (female Gamzee) went insane and killed Karkit with a club to the head. Nepeto then, out of revenge, stabbed her in the stomach and killed her, then desperately tried to revive Karkit to no avail. He left and was deeply distraught. Somehow Karkit was able to be revived later, and now he is much happier but still scared of some Gamzees.
Pretty much the same as Nepeta. He can be fairly rude at times and as mentioned earlier his emotions are a bit more extreme.
Karkit Vantas's Matesprit. He loves her to death. Literally.
Nepeto has an entire section of his cave hive that has been drawn on the story of their love. They often have fights over who can make the biggest heart or who can make the best thing to show their love. Of course he won when he painted on his cave wall.
The same as Nepeta, he has Pounce De Leon. But it's male and has a slight mane. He likes his Lusus a lot and will often sleep next to it when he's at his hive.
He also has a kitten named Marco Pawlo (Marco for short) which was given to him by Karkit. It's still a very small baby kitten though.