Kotarou Kashima is one of the main protagonists and is the younger brother of Ryuuichi. He was said to be voiced by Diana Kaarina in the English dub.
Kotarou has short, light-colored hair and a sleepy look on his face. Occasionally, his eyes sparkle whenever he's excited or happy about seeing his brother or something intriguing like hatching chicks. Like the other kids in the daycare, his cheeks appear to blush whenever he's curious or about to burst into tears.
Kotarou often wears clothes with a small picture of a tiger (relating to his name which means 'tiger'). At other times, he's also seen wearing a panda hoodie.
In the daycare center, Kotarou's often seen reading a book quietly in one corner, or requesting Ryuuichi to read him one of his choice.
Kotarou's quiet and well-behaved, or on some occasions being the only child at the daycare to not cause trouble for the babysitters. He's very attached to his older brother, and sometimes he gets upset when the other kids hold his attention. He often goes out of his way to ensure Ryuuichi's happiness, to the extent of ensuring that his brother is well-liked by others. For instance, when Inomata's mistaken as someone who dislikes Ryuuichi, Kotarou appears to be dejected and he's encouraged by Kirin to show his brother's charms to comic effect. He appears to be fond of the chairwoman and calls her "Fluffy" ("Shaggy in the anime), referring to her big hair which it makes her resemble a sheep.
Kotarou's well-liked by the other kids and he's very considerate of their feelings, on one occasion choosing to stay overnight with Usaida to avoid picking one friend over another. He appears to dislike conflict in general, and he tries to mediate an argument between Kirin and Taka but it doesn't work.
Kotarou's also a strong and determined kid as he's seen by his attempts to make lemon tea for Ryuuichi when he's sick, and climbing a flight of stairs to deliver Ryuuichi's lunchbox he forgot to get. In addition, he appears to be comfortable in the arms of strangers or acquaintances, and he trusts them easily. He doesn't cry easily, only doing so when he perceives Ryuuichi if he leaves him, or having his efforts go to waste.
In contrast to his painfully oblivious older brother, Kotarou is very observant for his age. He is able to perceive tiny details, such as ants, that go unnoticed by Ryuuichi or others.
Kotarou has one brother, Ryuuichi, and his deceased parents who died in the plane crash. After the incident, they're adopted by Youko Morinomiya, the chairwoman of Morinomiya Academy who offers to act as their guardian until Ryuuichi's an adult on the condition that Ryuuichi makes use of his experience taking care of the kids in the school's daycare center.
In the past, Kotaro's shown to be a naughty kid who destroyed his brother's project whose personality is a little bit different. When Kotarou first arrives in the daycare room, he's very apprehensive about the new situation. Then Taka takes to him quickly and sees him as a playmate, or a "monster", which Ryuuichi quickly interferes for his brother to slowly adapt. Eventually, Kotarou gets sick and with Hayato's help, he locates a nearby pediatrician. Then Kotarou cries for his brother who's talking to the concerned chairwoman. The next day, Kotarou starts warming up by reading with the rest of the children, visibly in a better mood.
- The "Ko" in his name is a reference to the word tiger ("虎 tora").
- His favorite food is a sausage cut into an octopus shape.
- A running gag in the manga is the way the other children often sprint off while dragging Kotarou behind them, often fast enough to lift him off the ground.
- In Ryuuichi's mysterious encounter on the snowy mountain, Kotaro shares an uncanny resemblance to the ghost kid he met.
- In Taka's ranger game, he's referred to as the, "Ranger Blue".
- He always refer his older brother as "nii-chan" or "ryu-nichan"