How does pKa relate to pKb?
Through the Ion constant for water.
pKa is just the -LogKa of Ka and pKb is -LogKb.
Ka and Kb are just dissociation constants of what they represent (Ka for acid and Kb for bases)
Thus this means that a High Ka means a "strong" weak acid and a low Ka means a "weak" weak acid. And the pKa is the same but contradicting. High pKa means "weak" weak acid and low pKa means "strong" weak acid.
The same goes for Kb and pKb but just for base.
Ka and Kb are usually large numbers, and as to create smaller numbers we use pKa and pKb.
Ka can relate to Kb through the ionic constant for water (since water can be both a sour and a base depending on what you are working with). Since pKa and pKb are the same (but -Log form of) as Ka and Kb, they relate similarly.