From ErfWiki

[edit] Proposed Canon

"We should just blast him while he's talking."

Strengths: Ruthlessness.

Weaknesses: Respect for authority.

First Appearance: LIAB 82

Sean Mattox is a Carnymancer seen in the Magic Kingdom who opposes Parson's passing through Portal Park to get to a battle, but eventually defers to Jojo to let him through.

[edit] Real world references

https://plus.sandbox.google.com/108169111610901863311

"The Carnymancer in the brown suit and tie in Book 2 page 082 is Erfworld Kickstarter supporter Sean Mattox, whose generous pledge earned him the cameo as a caster in the Magic Kingdom." – WoT July 25th Newspost