Understading homonyms always requires the context. In that webtoon, I thought the word was 사자(嗣子) which means one who perpetuates the lineage. It's like an heir, although 태자 and 황태자 (For your information, 루블리스 카말루딘 샤나 카스티나 is described as 제국의 황태자.) are better to refer to an heir to the throne. 사자(嗣子) can also be found in Chapter 11 of an old novel, "운현궁의 봄", written by 김동인. This novel could be too difficult for language learners to read.
The thing was that 사자(嗣子) is not a commonly used word at all. Since the webtoon is not as old as "운현궁의 봄", 사자 is supposed to be either a lion or messenger. The picture has "사자에게 충성을." No one, however, will swear allegiance to a messenger. I've just read part of the novel version and noticed "황금 사자" (a gold lion) is used as the royal emblem. Hence, the word should mean a lion (獅子). If I am right, the lion indicates the empire or the king. "사자에게 충성을." is translated as (I pledge) Allegiance to the lion (emblem).