Russell is a 9-year old Wilderness Explorer.
Russell was missing the "Assisting the Elderly" Wilderness Patch, the last one to put him in the rank of Senior Wilderness Explorer.
He showed up at Carl's doorstep, hoping to help him in crossing something to assist him. Unfortunately, Carl tries to get rid of Russell by saying that there was a "snipe" bothering him and tells Russell to look two blocks down to capture it. When Carl sends his home (and him), in the air with 10,000 colorful helium balloons, Carl finds Russell holding on for dear life waiting for Carl (whom he kindly calls "Mr. Fredricksen") to let him in. He tells him no, and Russell is forced to wait 5 seconds before Carl finally decides to let him in.
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Russell makes friends with an exotic bird which he names Kevin, and then finds out that Kevin is really a girl, searching for her eggs (though he continues to call it Kevin even after the discovery). He also makes friends with Dug, a dog who can literally speak his mind, using a collar that his old master, Charles F. Muntz, made for him.