This is based on opinion. Please don't list it on a work's trope example list.
Tear Jerker / Ciaphas Cain
Cain giving his sash to the mortally wounded Donal in Cain's Last Stand. Especially because he wasn't actually mortally wounded and he survived to be mind-controlled by Varan. Later, he comes in contact with Jurgen which gives him enough clarity to apologize to Cain and shoot himself when he can't break the spell.
As Cain and his Commissar Cadets are evacuating the Schola Progenium, he looks around his surroundings and notes that the Schola, and in extension Perlia, is the closest thing he's known to a home. Given that he's gone at length throughout his memoirs about how unpleasant the galaxy could be, it's surprising coming from him.
Cain spends his entire life behind his cavalier facade and never makes any meaningful connections or fully entrusts himself to anyone, not even Amberly despite their century-spanning romance. A telling moment is his second conversation with her wherein she gently suggests that he could try being himself. He nearly shits his pants thinking she's a psyker and might have him shot for cowardice, completely missing her point. That's sad.