He's a cut above, it's as simple as that.
For someone who is slow off the mark, he doesn't get yellow cards. Normally, slower defenders get turned by pace, panic, lunge in and get a yellow or they sit so deep that they allow the opposition to get between the lines and play freely.
Neither of those situations ever seem to happen with Helander.
Also, he is seemingly always in the right position. You only really notice he's a bit slower than the rest over five yards when he shepherds a player out into a safe position instead of letting them inside.
For a second you think "oh no, they're faster than him" and then you realise that both Helander and the attacker are chasing the ball whilst literally running away from our goal.
Finally, my favourite bit of defending during the Old Firm game came from him.
Thumb jumped over the keeper, expected an easy tap in so didn't dive over (which he would have got a penalty for), looked up to see the angle for the tap-in and did a literal double take as Helander was there in his face shielding the goal.
He got so spooked by it, he hit the shot at the corner flag and was easily pounced on by a recovering McGregor.