The fact that Darren and Steve are much more affectionate in the manga.
The Vampire's Assistant.
That one Little Person who limps comes to help them bury Sam, when all the rest of his "brothers" wanted to eat him. Makes you wonder if that character will become an important friend to Darren later on... if only he had a name, or could talk, or anything like that.
The vampires reacting to Darren's adorable honest comments - Gavner and Larten when Darren comments on Gavner's underwear, Kurda when Darren responds honestly about him not being very muscular. The way all of the vampires joke together when they're not fighting.
During the scene when they first meet:
Darren: "How do you become a Vampire Prince?"
Kurda: "Why? Thinking of applying for the job?" he said with a twinkle in his eye
Darren: "Not really," I chuckled, "Just curious".
When Kurda takes Darren on a tour of the mountain, Darren praises his ability to make maps. Kurda is touched by this, as his fellow vampires consider them boring and unnecessary. Considering how the poor guy has been ridiculed for his pacifism and lack of inclination to be a warrior, it must have meant so much to finally have someone admire him for his knowledge.
Trials of Death
The vampires changing from betting against Darren in his trials to betting on him, and coming to see him as a hero. That is, until he fails...
Kurda helping Darren escape, and refusing to abandon him and save himself when they hear footsteps. He says, "You know I'd never leave you," — and he doesn't. As Harkat, he stays by Darren's side through everything, forming a bond with him that's second only to Darren's bond with Larten, if even that.
The Vampire Prince
The sheer amount of times Cyrus is pictured in the manga. He has more full drawings of him than there are words about him in the novel. He's even on a cover, and from that we know that his hair is blue. Kurda's other assistant, the tall, bald one, is also drawn several times. The book mentions three, and in the manga we only get those two, but we get a lot of those two, even into books 10 and 12, and it's brilliant.
Before Arra dies, she convinces Larten to spare Darren
The friendship between Kurda and Glalda, in the manga. Although they're from feuding races, Kurda does genuinely seem to care about Glalda, while Glalda clearly believes in Kurda's cause to unite vampires and vampaneze. It shows that Kurda wasn't alone in his crusade and Glalda was working on the vampaneze's side to achieve peace between both races. Too bad the arrival of the Vampaneze Lord forced the two to take more drastic measures, which resulted in their doom....
While it's mixed with Tear Jerker, Kurda shedding tears for his vampaneze allies and his assistants as he's executed in the manga. Even if nobody will remember their noble yet senseless sacrifice, he will.
When Darren becomes a Vampire Prince, he sees the spirits of Arra, Gavner and Kurda congratulating him.
Half heartwarming, half... kind of mean of them, but just imagine that hall full of vampire generals deciding to make Darren a Prince. All voted in favor. Think about the reactions they all would have had, and then thinking it over and deciding "why not?". The last rounds of "are we sure? Are we really going to go through with this? Any objections? Any at all? He's 20 years old. He's a half-vampire. Anyone? Alright, then... Okay, now, everyone, poker face. When he comes in, let's let him think we're still going to execute him for a bit, and then tell him."
And the first of the Vampire Princes to vote for Darren? Arrow. The Saga of Larten Crepsley reveals he married a human woman once and had adopted kids with her. Even though he became cold and detached after her death, he still has a soft spot for children.
The Lake of Souls
The whole explanation regarding why Kurda was made into Harkat: in order to protect and support Darren, even by masterminding his own downfall.
That explanation being Kurda's extreme impulse to protect Darren at all costs. All of the times Kurda/Harkat saved Darren... which is, pretty much any time he was around when Darren was in danger. Including the many times he tried to talk Darren out of getting himself into danger in the first place.
In the end of book 5, Kurda's ponytail snaps open at exactly the moment when he stabs Gavner. It's sort of a visual cue — ponytail Kurda = friend, hair-out Kurda = enemy (see also his trial in book 6). Just before Kurda walks back into the Lake of Souls, Harkat rips off a ribbon from his blue robes and uses it to tie Kurda's hair back for him. It's a nice, symbolic way of saying they accept him.
Darren addressing Kurda as "Sire" just before they part.
Sons of Destiny
In the manga; despite all the talk about killing Gannen if it meant bringing down the Vampaneze Lord, Vancha ultimately hesitates when he has the chance. When Gannen takes the upper hand and is about to kill his brother, he starts crying, showing that he doesn't want to kill Vancha either.
In the manga, when Darren is in the Lake of Souls, and he feels himself being pulled out but he doesn't want to go, and he resists... but then Kurda pushes him out. A last act of motivation from a lifelong friend.
With the War of the Scars over, Vancha and Gannen reconcile and despite their loyalties to their respective factions, they start working to unite vampires and vampaneze.
It's even more heartwarming in the manga, showing them shaking hands. And Gannen is smiling for the first time ever since he was separated from Vancha years ago.
Crosses over a little bit into Tear Jerker, but the reveal that Harkat is working with Vancha to unite vampires and vampaneze and has chance to succeed. So yeah, Kurda ultimately succeeded in his goal. It's just a shame that he wasn't around to see it and know that, in a way, he did make a better world.
The ending of the manga adaptation, portrayed above. Little Person!Darren is worried that Steve will still want to become a vampire even after changing the timeline....and then he sees Darren and Steve happily playing soccer together. Little Person!Darren smiles that they're still friends and happily disappears to the afterlife.
Even though in this timeline Steve was still discouraged by Mr. Crepsley and believes his mother won't miss him if he was gone, he admits he doesn't want to lose Darren and gives up on his dream to become a vampire. Aww.