What you have there is a relative clause—a clause connected to the main clause by a relative pronoun (who, whom, which, that, or whose).
There are two types of relative clause: the restrictive, and the non-restrictive. A restrictive relative clause provides a crucial piece of additional information; if you leave it out, the meaning of the sentence is changed. A non-restrictive relative clause, by contrast, provides information that is (presumably) of interest, but that is not vital to the meaning of the sentence.
A restrictive relative clause does not need commas, but a non-restrictive relative clause does.
My take on your situation is that there is only one Hero of the North to begin with, and it so happens that this hero has a partner named Michael.
If we leave your relative clause off, it does not change the meaning of the sentence; therefore it is a non-restrictive relative clause, and so it does need a comma offsetting it.
On the other hand, if there happen to be several Heroes of the North, and you are attempting to specify that you are talking about the one whose partner is named Michael, then it would be a restrictive relative clause. If that is the case, then you would not want the comma.
reference: Oxford Dictionaries article on relative clauses