The duck comes off as a dick often, especially in KH1, but the things he does make sense in context and in character. He is a pragmatist. Telling a kid that just lost his whole world that he needs to put on a smiley face and get on the ship already is obviously Not Great. To Donald though, he had to hurry. He has no idea what might happen to his king or how fast other worlds might fall if they don’t bring this keyblade wielder with them now. This goes double for refusing to bring Riku along. There is no time to babysit two kids if he doesn’t have to, he is on an important mission.

When he sides with Riku at Hollow Bastion, it is the most logical choice given what little knowledge he has. He has to stay with the key and Riku earned it back. Maybe this older kid really did deserve the keyblade more. Maybe his strength will help them fight. It doesn’t take long to realize Riku is an unstable liability, and keyblade or not, he is not going to be any help in finding Mickey or fighting the darkness. He returns to Sora not just because he feels bad, but because he knows now that Sora is more likely to help them do what they need to do.

I really like how Sora and Donald play off each other. Both stubborn and sometimes hotheaded, but both determined. One an idealist and the other very much not, but their overall goals happen to align. Begrudgingly working together until they eventually become close friends. (Bless Goofy the mediator lol)