Questions & Answers about Он говорит открыто и честно.
In this sentence, открыто and честно are adverbs, not adjectives.
- The adjectives are открытый (open, frank) and честный (honest).
- Their corresponding adverbs are открыто (openly) and честно (honestly).
You can see they modify the verb говорит (speaks), describing how he speaks. That is the typical role of adverbs.
They are not agreeing with anything here. The -о ending is simply the standard way to form many adverbs from adjectives in Russian:
- открытый → открыто
- честный → честно
- быстрый → быстро (fast → quickly)
- тихий → тихо (quiet → quietly)
So even though открыто and честно look like neuter singular short-form adjectives, in this sentence they function purely as adverbs and do not agree in gender, number, or case with он or говорит.