[QuikSand]

A little digging - no precision yet, but this should prove helpful. And you have to love the little twist.

On rookie player cards, the "fingerprint" section includes a mention of chemistry. It includes one of these things:

QB affinities/conflicts (judged against your current offensive team leaders)
Player potential affinity/conflict (judged against the position group leaders)
Chemistry followed by one of these words: Mellow, Cheerful, or Sociable

So... while it's fairly obvious, here it is:
The one-word description of chemistry for non-affinity and non-conflict players is a guide to their personality strength.

Mellow = low personality (I'm guessing up to 33)
Cheerful = medium personality
Sociable = high personality (67+?)

So, the cruel irony here is that the only in-game use of the personality value is in the chemistry area (affinities and conflicts)... and for rookie players, the text line indicating the affinity or conflict takes the place of the rough estimate of personality strength. i.e., the only time you'd actually care about the information, it's withheld from you.

Eat it, chemistry people.