During the Hogwarts holidays, Karn spent his time either dealing with matters at the magical ingredients shop or learning magic from Lupin.
His control over magic was an eye-opener for Lupin. This was the first time Lupin had learned that a wizard could perform far better without a wand than with one. If not for Karn's appearance, Lupin would have thought he was teaching magic to a magical creature.
As far as he remembered, only magical creatures needed no wands to use magic. It was their instinct.
In just one month, Karn learned quite a few useful spells from Lupin: the Disillusionment Charm, Apparition, Occlumency, the Patronus Charm, and the memory spell Obliviate.
However, Obliviate was only an introduction to memory magic. More advanced memory magic included Legilimency, which required passing a Ministry of Magic test before one could learn it. The requirements for Obliviate were not nearly as demanding.
Lupin did not know very much about Legilimency or Occlumency, either. He had his strengths and weaknesses, and memory magic and Transfiguration were probably his weakest areas.
As for Lupin's strengths, they were dark magical creatures and wandless magic. Knowledge of dark magical creatures was quite useful, and Karn asked him a great deal about them, planning to select some as teaching material for his Defence Against Dark Magic Class.
There was no need to ask Lupin about wandless magic. In that area, perhaps only magical creatures could teach Karn.
When Lupin cast spells without a wand, their power diminished a little. Karn was precisely the opposite: his spells were stronger without a wand.
Karn did not learn any dark magic from Lupin. Lupin was highly skilled, but somewhat inflexible. He certainly knew plenty of dark magic, but casting it had a great deal to do with the user's emotions,