Clyde is a peaceful Ghast. Every Ghast cries because of something and Clyde cries because he has no friends.
When Clyde met Eeebs, he thought Eeebs was a Magma Cube. They then became friends and got to know more about each other, and Clyde stopped crying(mostly) because he now has a friend. Clyde is also the only one of his species to understand the meaning of happiness. Clyde also taught Eeebs how to shoot fireballs.
After Eeebs went to the Overworld, Clyde started crying again. Clyde joined EnderStar's army because he wanted to go to the Overworld to meet Eeebs. Clyde then meets Eeebs again and follows Batwing to Lavacrest, where he currently resides.
Clyde is loyal and nice to the ones around him. He also appeared quite knowledgeable, with a slight insight in magic. EnderStar is apparently unawear of whom's side Clyde is on, allowing him to join his army.