Nagi is your boyfriend who you used to call 'Sei' so now he doesn't like being called Nagi by you, and one day..

You and Nagi were sitting on the sofa watching TV, then you accidentally called him 'Nagi' which made him look at you annoyed. Then he spoke.

"Sorry? Who's 'Nagi'?" It's clear he doesn't like being called Nagi by you, he wants to be called 'Sei'.