In one sentence:
Mastleen could be considered a Scientist, but his field of expertise is truly a difficult one: sentient behavior.

Career:
After graduating the Academy he served aboard the USS Atlantis, acting as Ship's Counselor, where he spent most of his life in Star Fleet. At the time when he asked for an extended shore leave he held the rank of Full Lieutenant, which he was allowed to retain. Official records say he went to Medara, in Betazed, but there's only rumors of what he was doing during those months. At this time he is the USS Lexington's Counselor, following orders to return to active duty as soon as possible.