This question makes me wish I knew more about english, because I understand the nuances of when to use guard/guardian, but not how to best express it.
A guardian refers more to the person and must guard something. Therefore there is also either an explicit or implicit indirect object. "I am the guardian of my children". A guard refers more to the profession or an action (verb). You can either guard something, or be a guard.
To be fair, guardian is not a very common term. The only time I ever see the word guardian outside of that Marvel comic/movie is on official government forms where it asks for "Parent/Guardian".
For instance: "The warehouse guard is guardian of the warehouse." Many would find this sentence, if not odd, one that they would not commonly say; hence, why "guardian" is not a very common term. More likely someone would rephrase this as "The warehouse guard guards the warehouse", replacing the definition of the person with the action she performs.