Kenzo Fukushima: Kenzo is 16 along with his friends. His name means Humble and Heathy, his parents gave him that name because they wished for Kenzo to be healthy. Personality-wise Kenzo is usually humble except for when he is excited or mad.

Daichi Aoyama: His name means Wisdom and Intellect, his parents gave him that name because they wanted him to be really smart, think ahead, and planning things out. But that doesn't take all the fun out when he's with his friends, he always has fun with them.

Aki Hirata: His name means Hero and Manly, his parents gave him that name because they liked superheroes... Akio believes he is super manly and tries to be a hero in some situations, for example, if a girl gets bullied by some guys he tries to jump in and be a hero even if he knows he's going to be beaten up, so you can kinda say he's selfless.

Haru Amari: His name means Light, and Sun, his parents gave him that name because they believed he could bring light to any situation if there is a lot of darkness.

So basically in the shortness,

Kenzo: 16

Daichi: 16

Akio: 16

Haru: 16

Kenzo is humble and healthy

Daichi is the smart one

Akio is selfless and thinks he's manly (he's not..)

Haru brings light to any situation.

So yeah...

Also, I chose those names to suit their personality.