Chief Librarian Varro Tigurius is set apart from other Ultramarines because of his Psychic Powers, and does not fully share in the brotherhood of the chapter. During The Chapter's Due he exerts himself terribly to keep endless daemon hordes at bay and feels the isolation keenly, near to the point of despair...until Chapter Master Calgar poignantly reaffirms his trust and friendship with him.
Prior to M'kar's final assault on Castra Tanagra in the same book, Calgar is tending to a dying Tigurius when civilians the Ultramarines found offer to stay with Tigurius. Calgar asks if they know anything about Space Marine biology, and their leader admits that they don't, but they promise to keep him warm and stay with him so he doesn't die alone. Calgar is fighting back tears at the simple kindness of the statement. Most of the readers probably are as well.
According to Warlords of the Dark Millennium: Sicarius an Inquisitor once tried to Tigurius for his powers; the entire chapter stood by Tigurius' side. When the Inquisitor pressed further, Cato issued a Trial by Combat. Even when his champion lost the Inquisitor tried to pull a gun on Tigurius; in a split second, Cato cut the Inquistor's hand off and told him to get out. Tigurius was always one of them in their eyes.
The way the Unfleshed, despite all their hardships, still bring food to the members of their tribe who are too weak to hunt, and the way these children, horribly mutated and thrust into a literal hell, still maintain their own shrine to the Emperor is quite touching.
In The Chapter's Due, after everything he has endured (betrayal by those he trusted, going into the Eye of Terror, and generally seeing the worst humanity has to offer), Uriel Ventris tells an Inquisitor of the Ordo Malleus that every single human life in the Imperium of Man matters, and that if they forget that, they are already no better than the hordes of Chaos. It's heart warming because in spite of all the cynicism pervading Warhammer 40,000, someone who has seen the worst in humanity still thinks they're worth fighting for.