So basically, you choose what you are the guardian of. There are different formats:

Yournamehere, Guardian of BLANK(s)
Yournamehere, Protector of BLANK(s)
Yournamehere, Guardian of all BLANKS
Yournamehere, Protecter of all BLANKS
Yourname here, sworn guard and loyal protector of BLANK(s)

Or invent your own! But please, at all times, keep it PG.

I'll start, Sensorfire, Guardian of all mooshrooms!