Jurgen proving his loyalty to the Commissar by volunteering for a suicide mission in For The Emperor.
"I'd like to go with you, sir. I wouldn't trust any of those frakheads further than I could throw a Baneblade, if you don't mind me saying so, and I'd feel a lot better if you'd let me watch your back" I was touched and I don't mind admitting it. We'd been campaigning together for the best part of thirteen years by that point, and faced innumerable perils together, but his loyalty never ceased to amaze me. Probably because the nearest I've ever got to the concept myself is looking it up in a dictionary.
When you consider that Jurgen, as a blank, naturally creates a sense of mistrust and dislike in others—and then there's his infamous odor—Cain's simple kindnesses were probably the first time in his life anybody treated him like a person. And despite any number of opportunities where Cain could have thrown Jurgen to the wolves (or tau, tyranids, necrons, cultists...), he never does. This time, methinks the commissar really doth protest too much...
Well, the first time they met Cain DID sort of leave him to be torn apart by a horde of Tyranids. He did come back to save him, though. (Albeit because he was chased that way by an even bigger horde.)
But in 'The Greater Good' Cain risks a shot that might send the whole installation they are standing in sky-high - because Jurgen's life is at stake.
It's quite subtle, but being responsible for the reconciliation between an estranged father and son in The Traitor's Hand.
General Kolbe swelled a little with paternal pride. I was to learn later that his youngest son's decision to join the praetors rather than the military had rankled for some time, and that the incident at the bridge had initiated a reconciliation that both would have been too stubborn to try for under other circumstances, so at least some good had come out of it. (Other than a pile of dead heretics, of course, which always brightens the day.)
Near the end of the same book Tomas Beije insults Colonel Kasteen in her absence. Cain's response? Challenging him to a duel.
"You can accuse me of anything you like," I said, playing to the emotions of the troopers with me with the ease of long practice. "But you will not disparage Colonel Kasteen in my presence. She's one of the finest soldiers I've ever had the privilege to serve with, and the regiment she leads is among the best in the galaxy." I holstered my pistol with what I considered to be a suitably theatrical gesture. "No doubt this farcical situation has warped your judgment, along with your manners. When you calm down I'll expect an apology on her behalf. Failing that, I'm sure we can settle the matter quite amicably on the dueling field."
It doesn't hurt that Cain is fairly certain that his opponent is comically outmatched against him. Cain is considered to be one of the best chainsawswordsman in the Imperium, his opponent, not so much.
The next time someone insults Kasteen, in The Last Ditch, it only takes Jurgen mentioning the challenge from last time to get them to stand down.
His decision to send back the wounded with an escort even though it means cutting his available forces in half in Caves of Ice. He might rationalize it with believing that the threat was over, but considering that the average Commissar would have just executed the injured for being dead weight, it's a welcome touch of human decency in a universe so often lacking it.
Near the beginning of the first book, Cain notes that one measure he instituted to expediate the merging of the two halves of the 597th was a weekly reward of an afternoon's leisure and an extra ale ration to the most efficient platoon in the unit. His subsequent lines are one of the few times he doesn't second-guess or deride one of his actions, a rare hint of true pride from him:
These days, I'm told, "Cain's Round" is still a cherished tradition of the 597th, and the competition for the extra ration of ale still hotly contested. All in all, I suppose there are worse things to be remembered for.
Slightly meta- Amberley Vail, despite all the gentle carping at Cain from her editor's high seat, does two incredibly dangerous things for an Inquisitor- she acknowledges on record that she associates with a coward and secondly, she's acknowledging this in front of a readership of Inquisitors. If it weren't for the fact that you feel that she's editing the books for Cain after his death as a final love letter to and for him, it would be Too Dumb to Live. Instead, it's ridiculously sweet.
Although if you compare some of the inquisitors in other 40k media working with Cain would only really offend the incredibly puritanical seeing as other inquisitors are known to work with daemonhosts.
Also note the reason for editing the Cain Archives: it is stated in the first book Amberley Vail's work is to get behind the eyes of one of the most celebrated HERO OF THE IMPERIUM!, so as to understand the conditions which might produce more of them...
Every single time Cain slips up and reveals he's not the selfish Dirty Coward he paints himself as (feeling guilty that he doesn't remember a guy who died under him eighty years ago, thinking a victory was due to enemy error rather than the considerable morale boost caused by his presence, trying to find a way to take out a tank swarmed by allied troops instead of shooting them and the tank, infact, even BOTHERING to think of saving the troops swarming an enemy tank instead of just blasting everything away, tank and troops, as Amberly noted was what a typical commissar would do, is likely what makes Cain a Father to His Men. Oh, and the troops go through it safely. Him on the other hand....).
In The Emperor's Finest snooty noble brat Mira (currently Cain's lover) snaps angrily at Jurgen when he brings Cain's tea in one morning (she's not a morning person herself). Cain quickly placates Jurgen's hurt feelings and as soon Jurgen leaves the room he tactfully but firmly defends him to Mira. It's startlingly sincere, especially when you consider how people normally react to Jurgen.
Cain: Please don't treat Jurgen like one of your household servants. He's an Imperial Guardsman, and the aide of a commissar, with an exemplery record of courage in the face of the enemy. He deserves a bit of respect.
Also in The Emperor's Finest, Captain Gries gives Cain the use of one of the Reclaimer's training chapels to exercise in. Cain is genuinely touched at the honor, considering that the Space Marines normally consider such places sacred.
Nearly a century later Cain can still recall every detail of his first sight of Amberley Vaill in holographic detail. He calls her the most fascinating woman he's ever met, and even says her companionship made the incidents of bowel clenching terror she got him into worthwhile. But he doesn't believe in Love at First Sight, oh no!
Furthermore, if you're paying attention, Cain more-or-less gives up his playboy ways in every (chronologically) subsequent novel after meeting Amberley. Who'd have thunk he'd end up being the kinda guy who settled down once he met the right girl? Not Cain himself!
It's touching how often he admits to thinking about Amberley in Cain's Last Stand. The planet he's on is in the direct path of a Chaos fleet. There are Tyranids in the asteroids and maybe Necrons but the scent of Hegentha flowers floating over a conference table reminds him that they are Amberley's favorite flower. And later after the invasion has begun the sight of a dawn sky makes him think of her eyes. Oh, yeah, he's got it BAD.
In The Traitor's Hand, Cain wipes out Slaaneshi cultists who crash an aircar into the conference room. General Zyvan, who's familiar with Cain after several campaigns says he's a hero and Cain comments on risking his life to save a Lord General:
Cain: The Imperium has a lot of commissars.
Zyvan: Not like you, Ciaphas.
Notable in that he calls an Imperial Guard Commissar by his first name, and also because really there AREN'T a lot of commissars like Cain. Unfortunately for the Imperium.