One of the possibilities when setting a password is to use non-ASCII characters, such as ¨þ¨ (that is a thorn). Well perhaps something a little more secure than just a single character.
But just how sensible is it?
The first thing to bear in mind is that you need to be able to enter the password reliably in all circumstances. A tale from the mists of time: I once set a root password on a Unix machine that included the ¨@¨ character, which normally worked fine but failed on the system console because on that terminal the old Unix tty was still active and ¨@¨ would erase a line, making it impossible to enter the password.
Fortunately I realised what the problem was before it became more than a little annoying.
But the point still remains – if you cannot type a password, you cannot authenticate. So for passwords such as firmware passwords, system encryption passwords, or normal computer account passwords, a password containing Unicode characters is probably a very bad idea.
But for when you have full control over your computer(s), such as for web account passwords, a password containing Unicode characters is worth considering.
So how safe is a password containing a Unicode character anyway? Well, on my usual password cracking machine, john the ripper is unable to crack the password ¨þ¨ in approximately 24 hours. Of course that is a bit of a cheat as john the ripper does not by default check Unicode characters, and if it did it would be able to crack a one character password. But it would take longer; adding Unicode characters increases the space that john the ripper needs to search in order to find your password.
And perhaps more importantly makes it less likely for a password guesser (Hydra for example) to be successful.
So if you normally use a password such as thistlethinthorn, changing it to þistleþinþorn is worth considering. Or indeed changing the separator between words in a multiword password to a Unicode character: thistle☠thin☠thorn, or red¡whistle¡wheel.