I don't remember if I ever had an opinion on this one, but I could see why you might be thinking ISFJ.
He seems to actually be pretty core in terms of "how to respect people" -- he respects his uncle, he respects Walt as an authority, he gives credibility to experience, he works hard, he is devoted to learning his craft and doing his best. He recognizes Walt is the best and defers to him even if it means admitting his own work isn't quite as good. He's actually pretty civil to Lydia despite having a hangup about her and failing to recognize how much she loathes him (because in general she keeps a veneer up with him); I would never imagine him trying to justifying overstepping his bounds with her simply because he wants her or can justify that she's not a nice person, he'd still approach her with respect. He was relatively non-descript, he was low-key, I wouldn't call him particularly brainy in the way Ti tends to get about itself. He's fairly straightforward and not overly complicated. He sometimes just does things without over-thinking them, although when he does it's usually to consider the people he has respect for because he "owes them respect."
(That one line in the finale always sticks out too -- "You really shouldn't have come back here, Mr. White," is all he says to Walt. Nothing mean, nothing emotional, just... regretful because he still respects Walter but basically the situation means there's not really any more out for Walter. And now business and formality means Walt has to go bye-bye.)
Basically, though, he has no real empathy with others. So it's all a matter of business, and anything is justified as long as he is fulfilling his obligations and/or doing business. He doesn't even flinch. Capping Andrea is a prime example: Jesse took advantage of Todd's kindness and also wasn't doing his job as expected, so Todd does what he hinted he'd do, in order to make Jesse do his job. He really doesn't even seem to be bothered by it. But he was extremely polite and low-key while doing so... that's the interesting part. Nothing mean or fancy, and it was relatively painless for her, she didn't even know it was coming. He didn't torture her. The inhumanity came in that she was just a part of Jesse's equation that he had to balance back out, so Jesse would remain true to his half of the bargain.
I can see why you might think he is an ISFJ.