Learn Spanish (Spain): El villano era fuerte y además invencible. - The villain was strong and, besides, invincible.

además means "besides" or "furthermore" and is used to add escalating information. While you can start a new sentence with Además (e.g., Era fuerte. Además, era invencible), linking the two adjectives in the same sentence requires y (and) to flow naturally, translating to "and besides" or "and what's more".

Could I use incluso (even) instead of además here?

Yes, but it changes the grammar slightly. If you change the phrase to "and even", the word y (and) must change to e because incluso starts with an "i" sound. So, you would say ...fuerte e incluso invencible. Using incluso also makes the escalation feel more surprising or extreme than simply adding a fact with además.

Why is the verb era (imperfect) used here instead of fue (preterite)?

You use the imperfect tense (era) because you are describing the ongoing characteristics or physical traits of the villain in the past. It sets the scene or background. You would use the preterite (fue) if you were talking about a specific, completed event, like "The villain was defeated" (El villano fue derrotado).

Would the adjectives fuerte and invencible change if it were a female villain?