Clay

Clay is a miner and the gym leader of the Driftveil City Gym in Unova. He uses primarily ground type Pokémon, with Excadrill being his strongest and favorite. The way Clay talks is akin to a southern accent, and he has a tough voice.

One of his bad habits is forgetting to lower the drawbridge between Driftveil and Nimbasa. This absentmindedness often gets him in trouble with Elesa.

Miss Magius turned Clay into an Abomasnow via illusion in October 2012, forcing him to temporarily close his gym to prevent his Pokémon from getting hurt (being ground types, many of his Pokémon are very sensitive to ice).

Since joining Twitter, notable gym battles Clay has had were with Emerald, Red, and three Nates.

Trivia
Clay's account started off as a seldom used Gallade with the handle "DawnOfTheGal."

The meaning behind the handle "ThisMineOfMine" is obviously meant to reference his profession as a miner. It was suggested by @Alxprit.