Boris Kropotkin is the father of both Ben and Didi and the husband of Minka, also the maternal grandfather of Tommy and Dil and the father-in law of Stu.
He, like his long-time wife Minka, comes from the "Old Country", so he says. This location was later revealed to be Russia. In the Rugrats Passover special, he mentions his wine glasses being from Smolensk, so it's possible that's the city he was originally from.
He always talks about how horrible life was in the old country however Didi once revealed that he actually comes from a very well-off family. She then added "He wouldn't know a goat if it bit him in the face". He frequently gets into petty arguments with Minka over various issues, such as who will turn on the television, and why chocolate cake should be used at birthday parties, instead of carrot cake. He is also a wonderful storyteller, as he would often tell vivid stories about "monsters". He and Grandpa Lou are a lot alike in story-telling, their overactive imaginations with their stories, and were romantic with their wives.
Boris is a very eccentric, yet interesting man who is able to tell great stories. He argues often with Minka and, occasionally, Lou. He usually rambles on about his past experiences living in "the old country".