He is cold, doesn't like to talk, doesn't care about anyone or anything and never shows his emotions. That's the character of the frost knight. In reality, he is a friend-who-is-not friend of the Sun Knight, and likes to prepare sweets for him. Creus even mentioned that he'll make a good --mother--father. He is a tough opponent, really good at swordmanship. He also hates to waste food.
Like Creus, he is trained from young by previous Frost Knight. Ecilan is a polite and caring child who keeps smiling and loves going to kitchen. Previous Frost Knight forces him not to show any emotion. Ecilan starts to wonder if he really can be Frost Knight and cries that it's too hard not to smile. He thinks that people will dislike him if he don't smile. Later with Creus's help, he realises that sweets can replace smiles and became good friend with Creus thereafter.