Learn Spanish (Latin America): Hay que respetar al adversario, el cual también es estudiante. - One must respect the adversary, who is also a student.

El cual is used here in a non-defining relative clause (the type separated by a comma) to add extra, non-essential information about el adversario. While quien or que could also work, el cual elevates the register to sound slightly more formal. Because it changes to match gender and number (unlike que), it is also a great tool for avoiding ambiguity in more complex sentences.

Could I use lo cual instead of el cual in this sentence?

No, because you are referring back to a specific masculine singular noun (el adversario), so the pronoun must agree with it. Lo cual is a neuter pronoun used exclusively when referring back to an entire previous idea or situation. You would use lo cual if the sentence meant, "One must respect the adversary, which [the act of respecting] is important."

Why do we need the al in respetar al adversario?

This is a reminder of the "personal a". When the direct object of a verb is a person, you must place an a before it. Even though adversario might feel like a generic title rather than a specific name, it still refers to a human being receiving the action of respect. The a then naturally merges with the masculine article el to form al.

Why is it just estudiante and not un estudiante?