The main character and namesake of the series, Signal is the newest HFR (Human Form Robot) to be created by Professor Otoi. He is unique because of the MIRA and SIRIUS technology in him, which sometimes makes him a target of those wishing to steal his data. He is programmed to be a fighter and an older brother to Professor Otoi&#039;s grandson, Nobuhiko. Due to an accident in the lab during his creation, when Nobuhiko sneezes, it transforms Signal into a much smaller version of himself. He is referred to as &#039;Chibi Signal&#039;, is very hyper, and adores chocolate.   

Signal, a robot companion built for Nobuhiko, is attacked by Pulse, one the its creator's previous prototypes. Reprogrammed by Dr. Otoi's mysterious rival, Pulse is loaded down with weapons and sent on a mission by Otoi: steal the most recent robot technology, and obliterate anything that gets in his way.
 
(Source: Anime News Network)

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__Non-anime roles:__  Takeshi Yamato in "IZUMO2 学園狂想曲" (Visual Novel)  

It follows the story of a humanoid created by a robotics expert named Dr. Otoi for his grandson, Nobuhiko. But in the process of writing and applying the programming, an unfortunate mishap takes place! Nobuhiko sneezes. Now, whenever Nobuhiko sneezes, the humanoid (named Signal) transforms from a temperamental adult robot into a chocolate-loving baby version of himself. 
Despite this little error, the days spent with the Otoi crew are usually fun. Animals, people, and robots all can live in harmony. 
Until one day, Signal is attacked by Pulse, a robot created by Dr. Otoi some time ago. Much like a prototype for Signal. Pulse is incredibly nearsighted, but makes up for what he lacks in vision with firepower. Dr. Otoi's mysterious rival has packed him to the brim with amazing weaponry, with the intent of stealing the Doctor's most recent robo technology. 
(Source: Wikipedia)

The sequel picks up the plot two years after the end of the battle with Dr. Quaser, and a think tank named At Random is attempting to apply robots to daily use for the public. The setting is in Singapore. The new story focuses on three characters: Lantse, a successor unit to Pulse; Maria Maria, a girl who is depressed after she suffers from an accident; and Shiruto, a boy who wants to become a robot. A 13-year-old Nobuhiko, a 19-year-old Chris, and Signal will also appear in the story.
 
(Source: Anime News Network)