Millie Syndrome 

Millie is a character that is married to another character named Moxxie
Both characters are from an adult animated web series called Helluva Boss that was created by VivziePop, which can be found on YouTube
Millie is commonly known as just "Moxxie's wife"
Many people's reasoning for this is that from their perspective, they do not see much of how she is outside of Moxxie, and most of the things she does revolve around Moxxie
So, when someone uses the term "Millie Syndrome", what they mean is that the character is severely underdeveloped and their existence seems to revolve around another character as a whole

"This character is suffering from severe Millie syndrome"

Millie Syndrome by MeatballDog February 3, 2024