Mr Stringer, like Mr Jenkins, is an adult character who's less than likeable. Mr Stringer, the manager of the Hotel Magnificent, is described as being a "bristly man" (5.41). This means he's kind of on edge and gets angry pretty easily. We see that right away in his conversation with Grandmamma. He's quite high strung and becomes raving mad when she suggests that there are rats in his hotel. Granted, Grandmamma is making the whole thing up, but Mr Stringer doesn't react very well.
This guy's also kind of pretentious – he thinks he's better than everyone else and is kind of fake in his interactions with people. On top of offering assistance, he bows – bows! – to the witches as he shows them to their meeting room. You know what's kind of funny, though? Even the witches themselves realize how much of a nut he is. The Grand High Witch herself refers to him as "that rrridiculous Manager" (11.12). This is maybe the one time in the whole book that we have the Grand High Witch's back.