Prior to the tithing Thumper came from Asteroid Sigma-Nine-Nine-Nine-Rho-Six-Five in the belt.  The site of where there had been a pirate incursion that was barely even spoken about due to all the redaction and clearance codes slapped on the event by both the PDF and the Arbites that had been sent in to deal with the issue.  The survivors...every man, woman and child, all enlisted in the 5th.

He doesn't talk about what happened, nor have any of the others.

Beyond that at the campfire he is more than jovial to banter and due to his unique form of courage, or madness, or faith in the God-Emperor he doesn't even have issues speaking to Guntram.  It was a iron will that paid out in spades during the Ork campaign in holding the line and even going so far as when he was mis-issued (not according to the Departmento of course) the heavy bolter to fire every last round before picking up a Ripper gun from a dead Ogryn that had been attached to their section of the front and turning the charging orks into as much green mist.