First, "of my heart" simply means that something is associated with your emotions:
You are the love of my heart.
Also, when we use words like "heart" or "head" metaphorically, prepositions may not mean what you think they mean. Consider:
You really got in my head for a minute there.
That simply means, "You affected my thoughts," and (thankfully) has nothing to do with you shrinking and climbing into my skull.
As for "of my heart" vs. "in my heart", here's one way I might characterize the two:
- of my heart can be used with the definite article, to indicate preeminent personal feelings
- in my heart can be used with the indefinite article, to project a role
You are a diva in my heart. (means: in my heart, I regard you as a diva)
You are the diva of my heart. (means: in my heart, I regard you as the diva)
So, back to what you said:
"of my heart" sounds like you have several hearts, and one of them is your champion...
the champion of my heart
means something more like:
there may be several people who could be my champion, but, in my heart, you are my true champion
Disclaimer: I wouldn't regard this as a hard-and-fast rule with zero exceptions. As was mentioned, prepositions are very tricky. Macmillan lists more than 20 possible usages of the word of, and when you combine those with metaphorical words such as head, heart or skin, some preposition-noun combinations will indeed be idiomatic, and need to be learned one-by-one (such as, "She really gets under my skin.")