Current questions and answers regarding the map editor.

How do I make a neutral hero?
Because when I change it in the tree, it disappears.

You can't do it directly. But... If there's an unused player slot on the map (let's say PLAYER_8) that hasn't been edited at all, it will also have a neutral color.
You can place a hero from an active player on the map in the editor, and then use a script to change its owner to the unused player.
SetObjectOwner('Numys', PLAYER_8);
Then the hero will actually become neutral.
(This example refers to the map RedHeavenHero "The Veil Falls" - the mage Fazim).