Markl is Howl's apprentice in the movie. Not much of his background or past is revealed, except for his first name and that he is training under Howl to become a wizard. He manages Howl's affairs while he is away, dealing with customers in the different towns linked to by the portal.
The only notable magic he ever does is put on a cloak and take on the appearance of a short, old wizard with a large beard. His voice gets huskier and deeper in this form.
Initially, he is suspicious of Sophie's presence in the castle, addressing her as 'Grandma', but he eventually forms a close bond with her as they are together every day. Most of his time is spent on shopping for food with Sophie, cooking, and other chores such as laundry.
I can't remember the last time we had a real breakfast!
Howl's apprentice in the book is Michael Fisher, a 15-year-old orphan from Porthaven who willingly stays with Howl and eventually becomes his apprentice, unlike Markl whose background information is unknown.