Though the mouse is the hero of this fable, the lion showed an important character quality which, in the end, saved his life. That quality is

The characteristic of the lion that saved his life in the end was mercy.

Explanation:

The story is about a lion and a mouse. In this the mouse wakes up the lion while he was sleeping, which makes the lion angry.

The mouse demands forgiveness from the lion and convinces him by saying that such an unworthy prey won't give the lion any kind of honour. The lion gets convinced by this and let the mouse go. This character of lion shows his nature of mercy which in the end helps him to save his life.

When the lion gets caught up in the net of hunters, the mouse remembers the lion's mercy and decides to set him free from the net. He then cuts the net with his sharp teeth and thus the lion was saved.

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d. cassius cannot believe that a weakling like caesar has become so great.