Kashima Kotarou is one of the main protagonists and is the younger brother of Ryuuichi.
He has short, light colored hair, and has a sleepy appearance. Occasionally, his eyes sparkle whenever he is excited and happy about seeing his brother or something intriguing like hatching chicks. Like the other children in the daycare, his cheeks appear to blush whenever he is curious or about to burst into tears.
He often wears clothes with a small picture of a tiger (relating to his name which means 'tiger'). At other times, he is also seen wearing a panda hoodie.
In the daycare centre, he is often seen reading a book quietly in one corner or requesting Ryuuichi to read him one of his choice.
The illustrator Hari Tokeino said that in the future kotaro and taka, can be good friends or more than friends.
He is quiet and well-behaved, on some occasions being the only child at the daycare to not cause trouble for the babysitters. He is very attached to his older brother and sometimes gets upset when the other children 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 is mistaken as someone who dislikes Ryuuichi, Kotarou appears to be dejected and is encouraged by Kirin to show his brother's charms to comic effect. He appears to be fond of the chairwoman and calls her "Fluffy", referring to her big hair which makes her resemble a sheep.
He is well-liked by the other children and is 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 tries to mediate an argument between Kirin and Taka but to no avail.
He is also a strong and determined child as seen by his attempts to make lemon tea for the sick Ryuuichi and climbing a flight of stairs to deliver Ryuuichi's forgotten lunchbox. In addition, he appears to be comfortable in the arms of strangers or acquaintances and trusts them easily. He does not cry easily, only doing so when he perceives Ryuuichi to be leaving him or having his efforts go to waste.
Kotarou has one brother, Ryuuichi, and deceased parents who are lost in the plane crash. After the incident, they are adopted by Youko Morinomiya, the chairwoman of Morinomiya Academy who offers to act as their guardian until Ryuuichi's adulthood on the condition that Ryuuichi makes use of his experience taking care of the children in the school's daycare centre.
In the past, Kotaro is shown to be a naughty child who destroyed his brother's project whose personality is quite different. When Kotarou first arrives in the daycare room, he is very apprehensive about the new situation. 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 falls sick and with Hayato's help, locates a nearby pediatrician. Kotarou cries for his brother who is 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" (Tora) in his name means "tiger".
- His favourite food is sausage cut into a shape of an octopus.
- He appears to be floating in the air when pulled by Taka or Kirin.
- In Ryuuichi's mysterious encounter on the snowy mountain, Kotaro shares an uncanny resemblance to the ghost child he met.
- In Taka's ranger game, he is referred to as "Ranger Blue".
- Kotarou is voiced by Megumi Takamoto on the drama CD in May 2011.
- In the anime, he is voiced by Furuki Nozomi.