I create look on main player, then copy the character string to a companion. It seems that the ability to customize companion isn't there right away, have to fight with them for awhile then the menu panel to do so becomes available. I don't know the precise condition.
You can always customize Followers but some have like fixed settings for example you can't change name of them or even spawn or have like another custom character which would be cool to have a full squad of custom characters