So his sister is essentially trapped in the spirit realm and comunicates with him in the physical realm?Since he is mute, I guess that his sister will do the speaking for him, I wonder though if he himself will have quotes similar to Sona or not.(after all we mostly hear the champs inner thoughts, so it would make sense to be able to hear him).
Overall I'm curious how he and Diana will interact, the lore of Aphelios makes him and his sister the figureheads of the lunari, but Diana herself is the literal aspect of the moon, meaning that Diana would stand above them in the cult, which could cause some interesting scenarios, maybe they are actually the ones who'll drive Diana into her zealous self that we see ingame.
Sadly his lore doesn't really go into detail about his personality besides "zealous lunari", so its hard to say how he'd react to these events(granted it was said that the noctum numbed Aphelios to everything, so he'll likely be pretty aloof).
And to be honest, he looks really feminine, I wasn't sure if he was a guy or a girl when I first saw him.
EDIT: I'm also kinda glad that we are finally getting a targon champ who isn't an aspect or even closely related to them. I mean sure the aspects are important, but its not like every targon champ has to be one.