Donald "Don" Eustis (pronounced "you-stis") is a minor character in the series. He is noticeably overweight and is Nate's neighbor, along with Francis Pope. He is the owner of Spitsy. He is very kind to Nate and pays him money whenever he helps him (rake leaves, walk Spitsy, etc.).
Mr. Eustis is a very kind and understanding individual. He is also highly sympathetic. He is a divorcee, like his neighbor, Marty.
Big Nate On A Roll
Mr. Eustis is first seen ordering Spitsy to disengage after the latter pounces on Nate and begins licking him mercilessly. He then asks Nate why he was at their house, and Nate says that he is selling wall hangings for the Timber Scouts. Mr. Eustis agrees to buy one, and Nate promises it will come to his house in three or four weeks.
A few days later, Mr. Eustis learns that Nate means to raise money, and hires him to walk Spitsy for him. Unfortunately, Nate gets a black eye during the walk and ends up spending an hour tracking down Spitsy and Pickles. When Mr. Eustis finds out about this, he pays Nate to paint his fence.
Nate is friends with his neighbor, Mr. Eustis. At one point, Mr. Eustis buys a wall hanging to support Nate and the Timber Scouts, possibly because he felt sorry that Spitsy "used [Nate] as a tackling dummy." Nate usually walks Eustis' dog for him, whose name is Spitsy.
Spitsy is Mr. Eustis' dog, and he appears to be very understanding of him. It is likely Mr. Eustis knows Spitsy is in love with Pickles, though this is not confirmed.