Himitsu no Lunch

Koharu is a petite and thoughtful administrative assistant in the office. Katsuura is a big, hard-nosed sales rep. These two elite employees from different departments usually don't get along... Until one day late at work, when Katsuura finds Koharu eating by herself in the office, with a massive bento spread across her desk — very different from her cute image...?

(Source: Kadokawa, translated)