I'm fine with just calling him Key. After all, he opens all the possibilities.
It's hard to not fall in love with the guy and I'm pretty sure he's special in the Don/Dame vein. The only thing is his defense needs some work, but he's also only 19 and the effort is there. Once he's had some real time in training camp, film, and what not, I suspect he'll be fine there. But even if his defense is flawed, it's hard to ignore +25 despite that.
If I'm Ainge and Hardy, I check with him on if he's good with shutting it down. There's not much left to prove really, but also don't want to dim his fire.