Unless you use the first

I am full, never blank

Sometimes I make you burst

I am surely one of a kind

The ruler of a formal wear

A man of wisdom you might find

In your hand I'm very fair

I have three good friends

All with swords in their hands

Our bond never ends

But I am different in thy lands

What am I?

Answer to last week's brainteaser

1) Water/Fire: WfATEireR

2) Ground/Sky: GRsOkUyND

3) Rich/Poor: RpoIoCrH

4) Freeze/Boil: FRbEoiEZlE

5) Pen/Pencil: PpENeCInL

6) Woman/Man: WmaOMnAN

7) Light/Dark: LdaIGrHkT

8) Over/Under: OunVdERer

9) Clean/Dirty: CdiLrEAtNy

10) Right/Wrong: RIwrGoHngT).