Learn Turkish: Bu karakter zayıf ve bence korkak. - This character is weak and, in my opinion, cowardly.

This uses the suffix you are learning right now: the equative suffix -ce. By attaching -ce to the pronoun ben (I/me), it literally means "according to me" or "from my perspective".

Is korkak related to the verb korkmak (to fear)?

Yes. The suffix -ak (or -ek) can be added to certain verb roots to create an adjective or noun describing someone prone to that action. Since korkmak means "to fear," a korkak is someone who is fearful or "cowardly."

Why isn't there a verb like "is" at the end of the sentence?

In Turkish, the third-person present "to be" suffix (like -dir) is often dropped in everyday language. Ending the sentence with the adjective korkak is perfectly enough to mean "is cowardly."

I've seen zayıf used to mean "thin." How do I know it means "weak" here?

Context is key! While zayıf is the standard word for a "thin" or "skinny" person when talking about body weight, its direct meaning is "weak." Since it is describing a person's character alongside being cowardly (korkak), "weak" makes much more sense here.