Fill in the blank with the correct form of the imperfect tense. tú (ser) guapo.

In Spanish (Castilian) the imperfect tense is used to:  

- talk about past habitual actions  

- describe what someone was doing when they were interrupted by something else.

In other words:

Now, the imperfect tense of the spanish verb estar (to be) with the 2nd person in singular tú (you) is estabas (were being).

Therefore the complete sentence to indicate that someone was being handsome is:

You were handsome