I saw Jackson in that video conference interview today, where he explained why he didn't work out with the Celtics initially, when they had the #1 and everybody was saying Fultz was the pick. Later, when they traded the pick a few days ago, he said it was too late to arrange for a meeting before the draft, but he liked the Celtics and would love to play for them. He said he had talked several times with Jaylen Brown, and thought they could play together very well.
He said his biggest attribute is toughness, and being a team player. He isn't one of these shoot first types, and he considers himself a defender.
My impression is he is a tough dude, along the lines of Smart and Crowder. He and Smart on the court together could terrorize an opponent. But with guys like that, you need another scorer to go with IT.