Kishou Arima. Regardless of whether her feelings are returned or not, the man ghouls fear as the White Reaper has in truth held Eto’s heart since she was fourteen years old. He was the first person (aside from her fathers, but that goes without saying) to see and accept and value her for all that she was, all that she had ever been, and all that she could ever become. He was her first friend, the person she can talk to about anything. He is her partner in all things – well, in all things that matter unlike her feelings for him. He is her best friend, her love, her other half, her King. He is her be-all end-all, the sun in her sky and the air that she breathes, the flesh and blood and bone she requires to survive.
((This of course is just the “default setting” for the blog, the way she feels for all Arimas across the board, whether she says anything about it or not. There are other serious relationships in Eto’s life, but those are specific to certain muns/muses.))