The extreme interest in the character doesn't sound all that unusual to me - my DS had a long phase of being that interested in Thomas the Tank Engine and friends, which fortunately for him is pretty common for boys here! I think if I were you I'd focus more on getting myself out of the loop than on changing her interests. My DS accepted pretty easily that adults had limits to their willingness to be involved; I realise (now!) that we were lucky in that, but maybe there are ways to get that across? Maybe even ham it up: "Oh no! DD wants me to play stuff about X again! I know DD loves playing things about X but I'm going to explode if I have to do that again!". She needs to know that it's fine for her to be interested in things you're not interested in.

Email: my username, followed by 2, at google's mail