Bradley and Esther were paired together for the egg project in the episode "Follow That Egg!".
In South Park: The Fractured But Whole, a picture of Esther can be found in Bradley's bedroom.
As many fanfiction writers portray Esther as Kevin Stoley's sibling, and Bradley is one of Kevin's friends, this further suggests interaction between the characters.
There are very few fanfictions that include this pairing, and relatively few fan arts as well.
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