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:
It is specially used to describe what used to happen or what was happening.
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:
Tú estabas guapo
You were handsome