Midori Akagi, the younger brother of Retsu Akagi from Gunma Prefecture, serves as the fourth and youngest member of the KiraDeco 5 under the codename "Smart Green." Possessing strong combat skills, he nonetheless struggles to maintain focus during missions. His striking appearance features vibrant green hair and red eyes. He typically wears a light blue jacket over a light teal-green shirt, plaid shorts, and light brown boots, often adding a light bluish-purple scarf.

Serious and prideful, Midori deeply resents being treated as a child, especially by Retsu. A long-standing rivalry fuels this animosity, stemming from Midori's perception of Retsu's superiority and his own resulting feelings of inadequacy and jealousy. A pivotal moment cementing this hostility occurred when Retsu left him alone in a forest during a Gunma crane dance festival. Despite his initial abrasiveness, Midori reveals underlying kindness, notably comforting his Jewelpet partner Labra when they were trapped together in a cave.

Midori joined the KiraDeco 5 alongside his brother, becoming Labra's human partner. He moved in with Labra and Angela to study magic, though his intellect often leads him to lecture them instead. His character development involves reconciling his rivalry with Retsu, admitting his own mistakes, and gradually opening up emotionally. A subplot sees him briefly pursue Charotte romantically, using her honey to gain affection, but she ultimately rejects him and develops feelings for Retsu.

His journey culminates in a strengthened bond with Labra, enabling him to receive the Green Labradorite Jewel of Fate and achieve a new transformed state. By the series' end, he returns to the human world with Retsu and begins junior high school. Thematically, his name "Midori" (meaning "green") aligns with his color motif and codename, while his surname "Akagi" combines "red" (赤) and "castle" (城), contrasting with his brother's red-themed identity.