I have never met Josef, but according to git it has been 685 days since his first patch was accepted into the Koha code base. He now has 42 patches with a total of nearly 3000 lines changed, which in itself is a great achievement. But even more importantly, Josef is a committed tester. He is currently leading the number of sign offs for April, and is second only to Marc Véron (unsung hero number 31) in terms of sign offs for 2017.
Jo Ransom met Josef in the Czech Republic while on her Koha world tour and speaks highly of him. If I have this correctly he works for a University that has been using Koha for quite a few years now. It is so great to see users becoming contributors also.