He likes the color green and robots and may be considered an otaku. He is a fun loving and determined little boy about the age of 8 (In third grade). It is also shown throughout the series that he loves his big sister Mirai.
His appearance in the show consists of a green shirt with khaki shorts, he also carry a black backpack. He has brown eyes, a very small nose, black hair, and is almost always seen with a smile.
The boy meets an unfortunate fate near the end of the series. Yuuki falls when trying to take Mirai's hand, and though Mari assumes he is either suffering from a cold or heat stroke, it is suggested at the very end of the series that Yuuki's death was caused by the head injury he suffered when saving Mirai from the collapsing Tokyo Tower. This could have caused his brain to swell and ultimately kill him, but as Mirai says, he never likes to show when he's sick; he passes off his condition as a stomachache before his final collapse. Mirai and Mari take him to the hospital, but the doctors are unable to save him. It is never revealed which of Mirai's nightmares actually happened, only that Yuuki did, in fact, die, rather than only needing an IV. The viewers see him again, but it is only his spirit promising to make it home with Mirai.
Yuuki is the boy is this picture below.