>>8857667
> an alien race compelling within a story
Compelling character(s). Unless the story takes place from an alien perspective, every xenos is basically an ambassador for the reader/player. Interesting design is good too. Not of a fan of weird features just for the sake of being weird though.
>What makes you invested in their existence?
Personally, I like knowing about long term desires and historic events that helped shaped them into what they are today.