Name a character that has a special way of indicating their mood.

Nagasumi's very own ahoge indicates his mood (and at the same time senses nearby, imminent danger) in My Bride is a Mermaid

Celty Sturluson (Durarara!) sometimes shows her mood with the shadow smoke coming out of her neck. Like when she was asked if she was in love with Shinra, the smoke puffed out shaped like a heart.