He was called Seven because his name also means: The Seventh Note of the Musical Scale,that is Nishad, or simply (Ni, नि ) in the musical Scale.
You might know,
स रे ग म प ध नि सं,
नि is Nishad.

Ms. Seven is a character from the story "Expert Detectives" in the NCERT textbook for Class 7 English. The story revolves around a group of children who solve a mystery with their detective skills. Ms. Seven plays a significant role in the story as the owner of a pet dog named Pluto.

Introduction:
In the story, Ms. Seven is portrayed as a worried and anxious woman. She seeks the help of the children to locate her missing dog, Pluto. The children, who call themselves the "Expert Detectives," take up the challenge to find the lost dog.

Appearance and Behavior:
Ms. Seven is described as a middle-aged woman with a kind face. She is deeply attached to her pet dog, Pluto, and is visibly upset by his disappearance. Her worry is evident in her actions and words. She is grateful for the children's assistance and cooperates with them throughout their investigation.

Interaction with Children:
Ms. Seven interacts with the children when they approach her to offer their detective services. She provides them with all the necessary information about Pluto, including his appearance, habits, and the circumstances surrounding his disappearance. She expresses her concern and hopes for their success in finding him.

Role in the Story:
Ms. Seven's role in the story is crucial as she sets the plot in motion. Her distress creates a sense of urgency for the children, motivating them to actively search for Pluto and solve the mystery. Her involvement also showcases the children's empathy and willingness to help others.

Conclusion:
In the story "Expert Detectives" from the NCERT textbook for Class 7 English, Ms. Seven is a worried woman whose pet dog, Pluto, goes missing. She plays a significant role in initiating the detective adventure undertaken by the children. Her character highlights the importance of empathy, cooperation, and problem-solving skills.