if there’s a German character, people tend to let them start sentences with ‘Ach’ or end them with ‘ja?’ and I think at least with the ‘ach’ it should be said that it isn’t used as an expression of disgust or a common starter for sentences as a whole. It’s usually used as another form of ‘oh’ as in surprise towards some new gossip for example.

So if you would use ‘oh!’ it would be a good place to put in an ‘ach!’ instead.