As stated above, Jim is the sensei in these classes. He even told Sissi that "only serenity will lead you to calmness". This is ironic considering that Jim is known for his fits of anger and his tendency to yell at the children. Sissi also states to Aelita (in Ultimatum) that she took Pencak Silat classes in a failed attempt to impress Ulrich, but whether or not she was referring to Jim's classes is unknown.