So...is it plausible to you all that Cullen might have had a good friend (Templar) that might have stuck with him through the rebuilding of Kirkwall and possibly joined the Inquisition as an agent/leader in our army?
I have a rather extensive headcanon that Cullen eventually helps Keran (from the Enemies Among Us quest) deal with the emotional/psychological repercussions of demonic entrapment/temptation in his life as a Templar, drawing from his own experience at Kinloch; and that leads Keran to grow fiercely loyal to him over time, standing with Cullen through the whole fallout of the Kirkwall mess and further (even through the Inquisition), as a sort of unofficial right hand to him. It might be because I really like Keran and think their story arcs are so similar that Cullen could easily see himself in that kid, but I think it's pretty fitting.
So yeah, I think your idea is very plausible. Cullen was a good Knight-Captain to his men at least, and had the right idea about the Order, so I'd think any good Templar in Kirkwall who didn't rebel with the rotten apples would have stayed loyal to him all the way through.