He has a mischievous and easy-going nature. He has a multitude of tentacles and pseudo-pods, all a mixture of green and black, but still has a recognizable outer layer akin to skin that secretes slime. The eyes have no fixed location. Since he lacks a defined mouth, or vocal chords for that matter, he communicates telepathically.
Boris forced him to free him from his prison and used him to camouflage himself. When both reached the dungeon heart, the insufficiently grounded wild magic fused their bodies together, much to the dislike of both. Later on Mercury struck a deal with the Baron Leopold, resulting in the separation of Boris and Tserk.
As a little known fact, "he" has no gender and reproduces by budding, which doesn't stop him from using the common misconception to scare or play with females. He prefers applying massages, though. Once he finally convinces Ami and Cathy to let him give them a massage, Keeper Midori ruins the moment. Of course, such interruption cannot go unpunished, and Tserk swears vengeance.
Ami uses him to rescue Jadeite from a monastery filled with priests, guards and the fairy sisters. Tserk accomplishes his mission using nothing more than a Wand that makes things go invisible, a flask of acid, a flask of base, some flour, some marbles, and two pills that transform whoever swallows them into mice.
While not very strong, he is incredibly dexterous and his preferred combat tactic is to simultaneously cycle six crossbows to lay down a barrage of long-range fire.