I don't think there's any single "right word" for this. Thank you for giving a specific example of the situation you are trying to describe. Honestly, "in the middle" is as good as anything. I might write the sentence as: "Based on the respondents' answers, KFC's consumer-brand relationship is neither weak nor strong, but somewhere in the middle." It is also reasonable to leave it out; having said "neither weak nor strong" there's no real need to say more: "Based on the respondents' answers, KFC's consumer-brand relationship is not unusually weak or strong." "Average" can work, too: "Based on the respondents' answers, KFC's consumer-brand relationship is about average, neither weak nor strong." "Based on the respondents' answers, KFC's consumer-brand relationship is about normal for a fast-food franchise, neither weaker nor stronger." Depending on the situations, words expressing something in between "weak" and "strong" could include "normal," "par," "average," "typical," "moderate," "fair-to-middling," "mediocre," "plain," "ordinary," but the choice would involve the overall phrasing of the entire sentence.

moderate

You can say that it is 'of average strength,' or 'moderately strong.'