Jenka was a bit-character in a badfic. He was a random zombie whose entire purpose was to attack the main characters and then "vanish." As a result, he appeared in the halls of Headquarters. There he wandered for some time until encountering Chalk, who tranquilized him (and much of the walls) when he lunged at her. A few hours later he awoke in Medical with a mind of his own and unable to infect anyone else. He was immediately assigned as Chalk's partner.
Since he was given little description in his home fic, his appearance varies. In all variations, however, he dwarfs his partner and is usually even tall enough to have to duck in order to not hit his head on the top of the door. He is also very strong and fast, easily able to carry Chalk and still make a mad dash for safety or a fleeing character.
Something to note: being dead, he has no healing ability whatsoever and has to manually stitch his wounds shut.
Jenka comes across as being overly serious. Truth is, he doesn't know what to do with a sense of humor so he sticks to keeping himself and Chalk safe. He only knows a few continua, so he usually relies on Chalk for fact-checking.