Kohaku is likely to be a being you would refer to as a "perfect angel". One of his most apparent traits is his naivité and innocence which make him seem very young and inexperienced. Kohaku has spent his life in heaven, among the other angels, and has therefore never been able to truly experience any world or life outside his usual surroundings. He is a very sensitive person who tends to be easily frightened and impressed. Kohaku is not capable of understanding the human world in the beginning, and does not know much about the values people on earth have. Money is something utterly foreign to him, and while he knows what it is used for, he has no idea how much it is worth. This is depicted very nicely in a scene of the second volume, in which Shuichiro's grandfather offers to buy something to drink for Kohaku, who declines, saying, that he does not want Shinichiro (the grandfather's name) to spend money on him.
This shows that Kohaku doesn't understand the value of earthly things such as food, drink and other items of the materialistic society. He is well aware that every human has to work in order to earn money, but he has no idea how hard somebody needs to work in order to pay for a drink.
Apart from his not realizing the low value/price of a beverage, there is also another aspect which often leads Kohaku to refusing to accept favors: his conscience. Being an angel, he both has to and wants to pay people back for helping him, and therefore is willing to do anything for Shuichiro after he has been saved by him in the very beginning of the tale. He now sees it as his duty to help out Shuichiro wherever he possibly can, and feels bad and guilty every single time he is not able to.
It is his goal to lead the most humble life at Shuichiro's house,never causing him any trouble or concern. It is his most earnest wish to pay Shuichiro back the favor and he is willing to wait until a time where he may get the opportunity to do so.
Kohaku is full of kindness , warmth and determination. He is also not capable of lying, and therefore comments on everything with great honesty. He has a lot of respect for not only fellow angels and humans, but even for animals and nature itself. He can communicate with trees and animals, and his love for every living thing in existance goes so far, that he would rather not walk on a field of grass, due to the fear of possibly killing insects and plants on the ground.
Seen from the platonic point, Kohaku surely is one of the most loving creatures you could possibly come across. He'll stand up for people and things he believes in, quickly leaving his often seemingly insecure nature behind. For example, he knows that Kouryuu is superior to him in almost every situation (Kouryuu's chibi-version is a lot more powerful than Kohaku's, and he has mastered many more techniques than Kohaku has), but nevertheless will fight against him if necessary. This proves that he certainly is by no means a coward and will go very far to protect things and people he deeply cares for.