Newt is a half-blood. We know his mother bred fancy hippogriffs, but either she is Muggle-born or his father is a Muggle. Newt has, as a result, grown up a bit more in touch with the Muggle world than any wizard in America could ever think of. Graves comes from an old wizarding family, and also from one of the most extremely segregated countries in the wizarding world. Show me all the ways this seriously impacts their relationship, and a few of the ways it doesn't.
Bonus points for:
- Other, subtler differences - the ability to appear to be a perfectly ordinary Muggle, for example, is much more important in American wizarding society than British.
- Graves isn't actively prejudiced against Muggles/No-Majs...but he's very rarely actually interacted with one for more than the five minutes it takes to Obliviate them, and as such is very, very uncomfortable about potentially offering a deadly insult to his fiancé's father/grandfather/Muggle aunts and uncles.