Jack is a big man (literally, at 6'10' he towers of most) in Dig Town. He helps ensure things run smoothly with the bazaar and daily amenities. If Dig Town had a recognized leader instead of a commune type atmosphere he would definitely be it.
This is a "special" NPC. As such he can be used slightly differently than others you have encountered. Players can "name drop" to temporarily raise Dig Town NPC's loyalty ratings.
When you use him name, the target NPC's loyalty rating is treated as if it were equal to Jack's loyalty rating for the encounter. If it is higher than Jack's loyalty rating, then it is increased by one for the encounter.
Be careful when you use this, though it is powerful it is a double edged sword. The amount of times you do this will be carefully tracked and can have negative consequences.