Tsunderes are characters who start out cold, emotionless, and heartless and turn into caring or at least able to feel emotions.

Winner for today is Decim from Death Parade.

In the very beginning, Decim is an expressionless and seemingly bored character. Throughout the series, he tries to understand human emotion, never really getting a true grasp on it.

Decim a wonderful example of a tsundere who grows slowly throughout the series.

Honourable Mention:

Sadao Maou/Satan Jason (The Devil is a Part-Timer!)

Germany/Ludwig Beilschmit (Hetalia)

Credit to the artists:
1. littlereddo from tumblr
2. Actually in the anime
3. aelanic from Instagram
4. Comment if you know them!