Neil is definitely the "weirdest" out of all the characters.  He comes from "far away" and is sent to Earth on a secret mission.  He also always wears a spacesuit and the helmet he wears sometimes changes color based on his mood.  Whenever he speaks he always says we (ie. we are not from here).  Also since he's an alien he's not "technically" a guy but everyone refers to him as he.  And since he's an alien he is very confused about how Earth and its people function.  How they speak act and all that.  Also since he's "technically" not a guy if any females want to audition as well feel free! Maybe I'll like your voice more than others.

So his voice is definitely the most "open" to interpretation I'd say.  The idea I had in my head was like a robot alien voice something higher pitched.  For an example, and this is kinda random, I thought of the announcer from the JackBox game Earwax (just a random video I found for an example).  He's not human so he wouldn't speak like a normal human since he is new to our language.  But that's just an idea I had that I liked, again feel free to give ur own interpretations.  Also, feel free to add some effects to your voice if you think it'd suit the character.  Just make sure you can replicate it again and try not to "overdo" the effects.