Learn Spanish (Spain): Mi exnovio nunca fue muy cariñoso. - My ex-boyfriend was never very affectionate.

You are right that the imperfect (era) is usually used to describe how things used to be. However, using the preterite (fue) here treats the relationship as a closed, finished chapter. It summarizes his behavior over that entire completed period rather than just setting a scene.

Why do we use a form of ser (fue) instead of estar (estuvo) for being affectionate?

We use ser because being affectionate (cariñoso) is being described as a general personality trait of the ex-boyfriend, rather than a temporary mood or state.

Why is exnovio written as one word? Shouldn't there be a hyphen or a space?

In modern Spanish, prefixes like ex- are usually attached directly to the base word without a hyphen or space, making it exnovio. You only use a space if the base concept is made of multiple words (like ex primer ministro).

Could I say no fue nunca instead of nunca fue?

Yes! In Spanish, if a negative word like nunca (never) goes before the verb, you don't need no. But if it goes after the verb, you must put no before the verb. Both nunca fue and no fue nunca are correct and mean the exact same thing.