okay this backstory shown that Sonya isn't in love with Millie. Sonya is simply very lonely. she doesn't have any true and long term friend before meet Millie. anytime she has friend, that people will leave her not long after. this is why Sonya is very scared to lose Millie too. although I don't know why Sonya doesn't believe on Millie's words. it's pretty clear that Millie never leave Sonya behind. both at the art convention and this international food arc, Millie always keep tag Sonya along.
oh and speaking of art conventions, I have to offers apologize to Millie. back then I thought she is either plain stupid or too naive to tag along Sonya and didn't makes it a date with Zen. after reading this chapter, I realized fatal error in my judgement. no, Millie isn't either stupid or naive, turn out she is really cares of Sonya and didn't want to leave her behind in fear that Sonya will be lonely again. I'm sorry Millie, you are a better person than what I thought before.......