Round characters are the characters that have complex, multi-faceted qualities which are similar to real people. These characters develop over time and are considered to be the focus of the story. Most of the time, these characters are the primary characters. However, other stories have round supporting characters.
For an example of flat characters, kindly look here:
Calor can't be an answer because it is not grammatically correct. If you were to use calor you would have to put esta haciendo calor.