Booster Dragon is the Rulishia's cute little dragon partner in the Demon Chapter. He has very large anime-style eyes, and makes very cute sounds at times. He also seems to be taken with wearing a rocket-pack as wears one to fly, mostly to aid Rulishia in air attacks. He can use his own wings to fly too, but he favors the rocket-pack as you might have guessed from his name and being a very young dragon, flying is difficult for him. Unfortunately, even for a dragon, he is not strong enough to directly assist Rulishia in battles.
As for his background information, he has been with Rulishia since she was a little girl. He doesn't physically look older since then but then again that could be because that was only a few years ago and dragons don't age in the same time intervals as humans.
His most adorable moment is when Carmine tells him to take care of Rulishia at the end; he makes a cute little salute. He probably could have been this anime's mascot if the series was made longer.