I started thinking, ghosts can’t be seen by people right?
Well, what if they couldn’t even see Teth but they definitely saw Billy stepping in front of the attack (probably some levitating weapons or something) and since Teth didn’t want to hurt Billy (the exact opposite really) he made it freeze inches from the ten year old.
The Justice League assumes this kid is telekinetic or something and want to train him because that is a powerful ability and his tendency of talking to himself is really easy to explain away as him having an imaginary friend. After all, it’s not unusual for kids to have imaginary friends.
However, once Zatanna joins the team Billy starts to really open up because while she wasn’t a necromancer like him, she did get magic. She even tells Billy the things the wizard neglected to tell him, because he kept forgetting that Billy was a total amateur because of how instinctual he was, and while she finds it odd that he can summon a hellhound but doesn’t know the very basics of necromancy she doesn’t say anything.
The two wind up forming a magic team and stick up for each other against the non-believer (Wally) and while Kaldur is always welcome to join, it’s usually just the two of them against everyone else.
And even though Wally doesn’t really believe in magic, whenever Billy needs to summon something quickly he does help him draw the pentagrams and Zatanna helps him out as well if he’s low on magical supplies