Nicholas is a very smart young man who has planned out this day very carefully.  He is dying to get into that lumber room.  There is nothing worse than being told you cannot go into a room.  He knows his aunt well, and he knows if he puts the frog in his bread and milk in the morning, she will arrange for the other kids to go someplace fun.  Then he will have the house to himself. 

He is a planner.  He knows where the key is hidden, knows how he is going to it down from the shelf, and knows how to open the door since he practiced on the schoolroom door.  When he opens that door, he has a wonderful experience because he is very imaginative.  While looking at the tapestry, he creates a whole scenario of what is happening in the picture. 

“But did the huntsman see, what Nicholas saw, that four galloping wolves were coming in his direction through the wood?...... would the man and his dogs be able to cope with the four wolves if they made an attack?” (pg 4)

Later we see that he is a quick-thinker.  When the aunt becomes trapped in the rainwater tank, she calls to him for help.   She promises to give him strawberry jam for tea, although she has no intention of doing so.  Nicholas knows that.  Nicholas says that he thinks she is the Evil One tempting him to disobey.  He will not give into the temptation.  He uses the subject of the strawberry jam to convince her that she must be the Evil one.  So he leaves her there in the rainwater tank until the kitchenmaid rescues her. 

He has a sense of humor.  The reader finds himself smiling at Nicolas’s ingenuity.  He has a spirited sense of fun, which I don’t think the aunt appreciated.

Why is Nicholas in disgrace in "The Lumber Room" by Saki?

In “The Lumber Room” by Saki, Nicholas is “in disgrace” for a number of reasons. The most obvious reason seems to be that he refused to eat his breakfast because there is a frog in it.

Nicholas was not to be of the party; he was in disgrace. Only that morning he had refused to eat his wholesome bread-and-milk on the seemingly frivolous ground that there was a frog in it.

There is an underlying cause for Nicholas's disgrace. The adults, who were supposedly older and wiser, tried to assure him it was impossible for a frog to be in his milk, but he knew better and proved them wrong. This was an affront to the adults' intellect. A mere child outsmarted them as he described the animal in great detail and turned out to be telling the truth. It must be noted that Nicholas put the frog there himself, so there was really no question of whether it was possible to have a frog in one’s breakfast. The adults, especially the aunt, found this action infuriating.

Who is Nicholas in "The Lumber Room" by Saki?

In "The Lumber Room," Nicholas is the story's protagonist, a boy who is creative and imaginative. He engages in acts of rebellion because he finds life with his aunt tedious and stultifying.

Clearly, Nicholas has a mind that differs greatly from that of his self-styled aunt, who reacts to his creative threat to her authority by arbitrarily rewarding the other children, his two cousins and little brother, with a trip to Jagborough sands. Undaunted by his punishment for having proven her wrong in her authoritative insistence that there is not a frog in his bread-and-milk--he knows because he put the frog there--the observant Nicholas informs his aunt of the reasons that the other children will not enjoy their day at the beach as he remains at home. (Later, Nicholas's predictions prove true.)

During the day, as Nicholas is prohibited from the gooseberry garden, he tricks his aunt into believing that he will try to defy her by sneaking into this garden.

It was clear to his aunt that he was determined to get into the gooseberry garden, "only," as she remarked to herself, "because I have told him he is not to."

Therefore, she spends her time guarding of this spot while he sneaks into the lumber room. There he delights in a tapestry and other works of art and pictures of nature's beauty that appeal to his own creative nature.

Later, Nicholas prevails further in his creative authority over his aunt because she has fallen into an empty rain trough in the forbidden garden and cannot get out. When she calls for help, Nicholas demonstrates his mental superiority as he reminds her that he has been told that he cannot enter this garden and, so, her voice must be that of the Devil tempting him. Thus, Nicholas leaves his aunt stranded, and she must wait hours until a kitchen maid in search of parsley comes to the garden and then rescues her.