Theory:

     “Try to understand, Seven,” I told him, “if he’s lived in Shankar House for a year and hasn’t made a single friend, there’s something wrong. He’s obviously scared that someone will recognise him and give him up to the cops.”

     “Maybe no one’s tried to make friends with him,” Nishad protested.

     “Why should anyone bother? You’ve seen what a nasty bear he is,” I said.

     “I don’t care,” said Nishad stubbornly, “I like him and I’m going to try and be his friend.”

     “Friends with a crook! Ha! You’re crazy, Seven,”I said. “The cops will take you to jail with him. Do you want that to happen, you idiot?”

     Nishad merely glared at me and quietly walked out of the room. My theories seemed to have made no impression on him at all.
Explanation:
   
Maya tried to persuade Nishad by reminding him that if Mr Nath had lived at the Shankar House for over a year and had developed no friends, something was clearly wrong with him. Perhaps, he was scared that someone would notice him and report him to the police.
 
Nishad retorted back by suggesting that no one would have tried to befriend him in the first place. That could be the reason why he didn't have any friends.
 
Maya was puzzled as to why anyone would bother becoming friends with Mr Nath. She then calls Mr Nath a "nasty bear", denoting the characteristic of a bear. Bear is seen as a solitary animal that largely keeps to itself. Moreover, it is also naturally aggressive. Hence, Maya states that Mr Nath, like a ruthless and unpleasant bear, doesn't keep friends and is quite hostile.
 
Nishad was, nevertheless, stubborn. He said he didn't mind Mr Nath being a nasty bear. Instead, he wanted to be his friend since he liked him.
 
Maya was quite shocked to hear her brother say that he would become friends with Mr Nath, whom she believed to be a criminal. Maya was both angry and worried. She was angry because Nishad had refused to listen to her. On the other hand, she was worried for her brother as becoming friends with a crook could be a dangerous move. She then warned him by stating that he would be taken to prison with Mr Nath if he ever got involved with the latter.
 
She then inquired as to whether Nishad would like the scenario to materialise. Maya's questions and theories had obviously taken no effect on her brother. Nishad was unconcerned about them, and as a response to her question, he merely gave her a glare and quietly walked out of the room.
 
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Nishad glares at Maya

Nishad was determined to make an effort into befriending Mr Nath. Though Maya's and Nishad’s journey into finding the truth behind Mr Nath's mystery was similar, their destination differed.
 
The lesson ends with Nishad walking out of the room, leaving Maya with her sheet of facts. Hence, the story becomes open-ended, and it is for the readers to interpret the ending of the story.
 
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In the end, Maya was left with her theories and her sheet of facts
 
Meanings of difficult words from the paragraphs:
  
Sl. No.
Words
Meanings
1
ProtestTo express an objection to what someone has said or done
2
NastyVery bad or unpleasant
3
StubbornlyIn a manner that shows a dogged determination not to change one's attitude or position on something
4
Bother
Take the trouble to do something
5
MerelyUsed for emphasising that something is small or unimportant
6
Theories
A set of ideas or presumed facts for the sake of argument or investigation
7
ImpressionAn effect or influence on the way someone feels or thinks
Reference:
National Council of Educational Research and Training (2007). Honeycomb. Expert Detectives:Sharada Dwivedi (pp. 126-132). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.