First of all, I will look at their similarities. Rikki and Nagaina both are more humanlike than the human characters and they both play bigger roles. The two characters both care for their own families and try to protect them. They also both try to kill the other animal's family to get rid of them. Those were some of the similarities between them.
Finally, I will now explain some of their differences. Rikki is mostly trying to defend his family