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 whilst 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. 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 for all of the 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!