Please help me to explain that why " could " is used here

Question

I was really tired of my old roommate. She could be so mean.

I thought that could in this sentence means was able to do.

Answer 1

This is a particular use of can (regardless of the tense, past or present):

You use can to indicate that something is true sometimes or is true in some circumstances.

• Exercising alone can be boring. (Collins)

In your sentence, we understand she could be so mean as meaning that she was mean, though not all the time. So here could is not focused on ability, but on expressing something real that happened at times.

Britannica says that can is

used to describe the way a person sometimes behaves

• She can be very rude at times. [=she is sometimes very rude]

used to describe something that sometimes or often happens

• In this part of the country the weather can change quickly. [=the weather often changes quickly]

Answer 2

There are at least three possibilities regarding the roommate's meanness:

1. She could be mean

2. She could not be mean

3. She could sometimes be mean

The word could indicates a possibility, but not an actuality. For example,

• I could say something negative at this point, but I choose not to.

• I could be at the top of my class, but unfortunately I lack the motivation to do so.

• Harold could go without eating for 48 hours, but he chooses not to fast [that is, to deprive himself of food].

The word can is sometimes a good substitute for the word could.

• The employees can work together as a team, but their boss is too insecure to let them work that way.

• Jim can sometimes be outspoken, but his wife encourages him to be silent.

• Cheryl can work independently, but when her boss micro-manages her, the quality of her work suffers.

Whether you use could or can, each word serves to denote a possibility, but not a sure thing.

• The weather could prove to be a hindrance to the successful completion of the project.

• The weather can sometimes interfere with working outdoors.