How do you describe someone who is always in the middle? For example, One person is always angry and another really calm and this person is both at times. Or they are shy and outgoing. Like science and art. Basically, they don't fit in a description cause they're always in the middle cause they do both or are both. Does this make sense?
Perhaps, that person is balanced.
1.3 (of a person or state of mind) having no emotion too strong or too weak; stable.
‘a balanced personality’
You could also use middle-of-the-road (Collins defintion):
- not extreme, esp in political views; moderate
Or level-headed (Collins definition):
- even-tempered, balanced, and reliable; steady
If you want to emphasize the fact that "they don't fit a description" as you put it, you could also consider unremarkable, although that's not exactly what you are looking for.
This was historically seen as a desirable characteristic, the idea that everything in moderation would be good. As such, it was referred to as the golden mean.
The desirable middle between two extremes, one of excess and the other of deficiency.
For example, in the Aristotelian view, courage is a virtue, but if taken to excess would manifest as recklessness, and, in deficiency, cowardice.