Niki (Nuka in Japanese) was a robot similar to Astro in construction, as well as Astro's first crush. Her primary appearance is in the 1980 series episode "Astro's First Love." Her Japanese voice actress is Mari Okamoto, and her English voice actress is Becky Wilenski.
Niki was originally built by Dr. Rindolph, a military scientist, after he stole copies of Astro's original robotic blueprints to make robot bombs. Astro is sent to investigate this while disguised as a human, but after jumping off a train near Rindolph's military base, he is spotted by Niki as she wanders in the woods. Niki helps Astro get into the base, walking in with Astro through the front gates. However, she begs Astro to stay with her, because she's never had a robot friend before. Astro gladly accepts.
Her creator, Dr. Rindolph, reveals to Astro that he has already constructed a robot bomb, and has put it inside Niki. Niki is horrified to learn this, and when Rindolph tries to launch Astro into the sun, she clings to his rocket. Rindolph is angered and decides to activate the bomb inside Niki's head. This leaves one hour until detonation, and after the children return to earth, Rindolph destroys his modified design right before Astro destroys the robot bomb stockpile. Astro begs Niki to come home with him, but she refuses, as she only has half an hour before detonation.
Astro forces Rindolph to stop the bomb, but the only way to do so is to completely dismantle Niki, with no hope of reconstruction now that the blueprints and robot stockpile are gone. Niki insists, as Astro initially protests, that she be dismantled, to save both of them and the rest of the people on the military base. Astro and Niki finally confess their love for each other together. During the dismantling, Astro holds Niki's hand, which eventually and sadly goes limp. While the bomb is safely and permanently disabled, Astro is extremely distraught.
The only solid piece of Niki left afterwards are her legs. Astro salvages them, and begs Professor Ochanomizu if he could replace his original legs with Niki's legs. Ochanomizu is initially reluctant, but agrees, helping Astro cope and to have Niki be a part of him forever.
Niki is very sweet, clever, and kind. She was Astro's first love and a big influence for him. Niki is neglected and forced to live alone in a military base, which makes her very open to befriending Astro. Niki and Astro build a very strong bond together and fall in love soon after.
Niki's character design was created not by Osamu Tezuka, but rather by artist and mangaka Haruhiko Mikimoto. Niki has blonde hair, along with brown eyes like Astro. She wears a red headband with small pearls with a red gem in the center. Her normal outfit is a long-sleeved red dress with a white apron, and red slippers. When her bomb is activated, the center gem in Niki's headband slides inward and glows.
Niki's primary appearance is in the 1980 series, in episode 51 (episode 52 in the Japanese order). However, she has a big role in the video game Astro Boy: Omega Factor as Duke Red's robot daughter. The credits to the game feature Astro flying through space, carrying Niki in his arms. The game calls Niki "Nuka", her original Japanese name.
Niki also appears, reimagined and slightly redesigned, in the 1987 American comic series. In this series, she is a human girl named Krystal Kleer, and is in Astro's class at the human school. Astro calls her "Kris", and at one point saves her from a dinosaur. Kristal is the first human child that Astro befriends, and she drops Astro hints that she has a crush on him. Her first interaction with Astro is when he gets harassed in class, and she writes him a note telling him to ignore his bully, and also that she finds him cute. When he has to travel to space, she gives him a flower as a souvenir.