She is a new red blood cell and AE3803's junior. She is introduced to AE3803 by AA5100. Despite her lack of work experience, she is well educated with the cells and body. She and AE3803 do not get along well initially due to their different personalities. While she did not have a high opinion of her senior initially, NT4201 still remained polite and civil towards her. This changed when the body suffered a hemorrhage and it's very survival was in doubt. NT4201 panicked and gave up, thinking all was lost. But thanks to AE3803's dedication and wherewithal even to the point of complete exhaustion won her over. After the body was saved by a blood transfusion, NT4201 admitted to being wrong about AE3803. But she eventually learns the values of work experience and takes her senior's teachings to heart.
(Source: Cells at Work Wiki)