Oh I see.
Objects are just specific types in LuaRT, like tables or functions. They can be used outside the ui module (for example : File objects, Zip objects...)
What you want is to display the string "-+-" at specific coordinates in a Window.
Just use a Label object like this, like you said :
local star = ui.Label(win, "-+-", x, y)
Another way is to create a new Object derived from a Label, called Fighter, with a specific constructor :
Fighter = Object(ui.Label)
function Fighter:constructor(parent, x, y)
ui.Label.constructor(self, parent, "-+-", x, y)
Then create a new "Fighter" instance :
local fighter1 = Fighter(win, 600, 400)