Aphelios doesn't speak because he consumes a poisonous substance called Noctum Essence. This essence empowers him with the abilities of his sister, Alune, but it also causes his throat to tighten, making it difficult for him to speak.
However, this doesn't mean Aphelios is completely mute. He can still speak when he isn't under the influence of the Noctum Essence.
The Noctum Essence plays a key role in Aphelios' relationship with Alune, allowing him to connect with her and use her powers. This connection is crucial to his identity and abilities.
