An ateva; any ateva, really. The 'Foreigner' series is my favourite bit of science fiction ever, hands down, no competition possible. The one time I ever for real as a fully grown person missed a bus stop because I was so fascinated by what I was reading? When 'Precursor' was newly out, I was reading it for the first time, and there was this massive reveal that subtly changed everything we had known about some of the atevi protagonists. I really, really care about them, and I would love to bring Banichi or Ilisidi, maybe even Cajeiri...

But atevi won't work on their own, and even with a number of associated atevi in Milliways, interaction with humans (and suchlike) would be hard, there would most likely be an incident of some sort which would end up with all the atevi in the cells and then banned, collectively, from Milliways forever. It's better that I don't even feel tempted to try.