Learn French: Cette agence immobilière est chère. Par contre, elle est très sérieuse. - This real estate agency is expensive. On the other hand, it is very serious.

Par contre introduces a contrast or a balancing point, translating nicely to "on the other hand." You could absolutely use en revanche here instead. En revanche means the exact same thing but is considered slightly more formal and elegant, while par contre is very common in everyday spoken French.

Does sérieuse just mean "serious" like a person who doesn't laugh?

It can, but when applied to a business like an agence immobilière (real estate agency), it means "reliable," "professional," or "trustworthy." The sentence is saying that despite the high cost, the agency provides excellent, dependable service.

Why does the second sentence start with elle to mean "it"?

Elle replaces the subject from the first sentence, cette agence immobilière. Since agence is a feminine noun, we must use the feminine pronoun elle to say "it is." We don't use il because that is for masculine nouns, and we don't use ce here because elle est is followed directly by an adjective (très sérieuse).

Why do chère and sérieuse both end in an "e"?