Nexus features in the Gamer route. When he first appears to the protagonist he seems like a rogue NPC in the MMO that the protagonist is playing. He's a little cold and suspicious, but ultimately comes across as someone who genuinely needs the protagonist's help, as he and his kind have been wronged. Nexus is quite righteous and although he looks like a child he acts like an adult. 

I selected ageless because his age is never mentioned and as I said, he looks like a child but acts like an adult, Therefore, a voice of either a child or an adult would be appropriate.

(Image is not final art.)