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
another thing you can't change is for example characters like the Coin Dragon that gets stats off gold can not be changed at all because over time the character is changing for example the bigger they get their legs slowly become digi
oh one final example if you customize the slim follower to anything it changes base color depending on potion so you can never hard change the color to a permeant one
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