At the end of "The Last Knight", Michael redeems Fisk and frees him so that he is not obligated to follow him around anymore and starts to leave. After a startled second, Fisk follows, complaining that if he'll only get into more trouble without him.
Anything in Rogue's Home that involves magic from Fisk's point of view. Considering that he's usually so sardonic it's sweet to see him being so sensitive to Michael's feelings.
Benton in Scholar's Plot. He's one of two people in Michael's family who haven't written him off as as a screw up even after Michael is disowned and pronounced unredeamed. He believes in Michael enough that when he needs help he, through Kathy, finds a way to get ahold of Michael who isn't even writing to Kathy and isn't with Fisk who does write to Kathy. Which had to take some determination. As far as we know he hasn't even told his own parents about his plight. But he tells his estranged baby brother.
Fisk's Character Development in Scholar's Plot, which is especially rewarding after he hits his breaking point at the end of Thief's War: he finally has a support system again, which he lost in Rogue's Home after his brother-in-law exiles him from his hometown, which was next to impossible to restore because of Jack's betrayal. On top of Michael, whose friendship with Fisk has been restored (and arguably improved), there is the mad jeweler who is the first person after Michael whom Fisk shows great concern for. But the most telling is Kathy, whom Fisk falls in love with. The book ends with Fisk declaring that he'll do whatever it takes to do right by Kathy, making his first conscious effort to stay away from a life of crime since his falling out with Jack Bannister.
Michael's character development as well. It's a bit more subtle than Fisk's at first but near the end, he's almost unrecognizable. He's still got that Incorruptible Pure Pureness thing, but he's much less idealistic. He sees the world for what it is, rather than what he wants it to be and still chooses to do good for the world, despite all the bad the world has done for him. But even more so, he knows how to take care of himself now. In the beginning, it's a miracle Michael managed to survive without Fisk, but in the end, when he goes off on his own with no one but a runaway apprentice you know he'll be alright. And all that is because of Fisk. He's taken Fisk's lessons of self-preservation and ran with them. The many times the two get separated in the last three books is proof that Fisk isn't the only one who has benefited from the other's company
Michael's reaction to finding out Fisk is in love with his sister. Fisk spends a good portion of the book, even before he realizes he's fallen for her, reminding himself that Michael's sister is off limits and after he realizes his love fo her he decides he's more frightened of Michael's reaction than her father's. However, Michael's reaction is simply irritation that no one bothered to tell him about it. It's sweet to see that Michael trusts Fisk so much that he'd leave his own sister to wed an ex-crimal.