Shirazu (and not Mutsuki) is the one Urie thinks about on Rushima Island, and how he wishes he were still there fighting with him and knew if he were still alive he’d have a better chance of helping Mutsuki against kakuja!Amon. His thoughts of Shirazu give Urie the strength he needs to continue on

This scenario is loosely inspired from the Trost arc of Attack on Titan. 

Whenever Urie feels close to collapse, Shirazu comes to him.

He can see him encouraging Aura to swing that way, right now. He would assure Higemaru he’s going great. He’d be speechless at Hsaio’s skill, and then he’d return to trading snark with Saiko.

He wouldn’t have let Mutsuki vanish.

Why did you go? As he spits out blood, Urie knows it’s not rational to hate Shirazu, but right now he does.

It’s not me. It’s yourself you hate. Shirazu laughs. 

Why? Why am I always losing you? Over and over and over? 

For a moment, Urie feels he will split in two if he uses his koukaku. He’s weak. He’s tired. He’s lonely.

But then he sees an enormous kakuja heading for Mustuki, and suddenly Urie is alive again. His koukaku is nearly weightless. 

It’s as if Shirazu is swinging with him. 

We have to live on, Urie thinks as his heart rate settles, because if we don’t, we won’t be able to remember you. 

In his head, Shirazu takes his hand.