Incomplete. Spoilers are unmarked.
Brandon cares for Harry deeply, and his devotion to Harry is easily seen throughout the series. Wherever Harry goes, he always follows him. To get Harry to the top of Millennion, he is willing to be a hitman, which is an Irony since he is terrified whenever somebody pulls the trigger of a gun and kills someone. Then Brandon, being part of Millennion, is supposed to enforce the Code of Iron and execute Harry by the time he knows that Harry is not doing the right thing anymore. However, he can't because Harry is his best friend and he actually chooses Harry over Millennion. When he admits it in the finale, he says that he has no regrets with his choice.
Harry's devotion to Brandon is harder to see, to the point it seems like Harry only subordinates Brandon all this long. Harry never cares about how Brandon feels during his days as hitman. Harry's rage over Brandon siding with Big Daddy can have two meanings: either Harry actually still cares about Brandon or Harry is just being a Manipulative Bastard. But when Harry suffers Villainous Breakdown and sheds Tears of Remorse after murdering Brandon, the former is more correct. Harry can't accept that he has killed Brandon and is actually just, for the lack of better word, jealous with Brandon seemingly to choose Big Daddy over him. Which is also why he is extremely angry at Big Daddy and decides to brutally kill him later, for the reason that Big Daddy takes away his most valuable treasure: Brandon.
The series ends with the reconciliation between Brandon and Harry. Brandon still shows his devotion to Harry even though Harry has killed Brandon's loved ones. Brandon actually can probably survive the ordeals if he has sided with the new Millennion by killing Harry, but he doesn't. He chooses to protect Harry from the agents. This, combined with his confession that he has always been choosing Harry over Millennion, has Harry begging for forgiveness and fixes their relationship. They then do Mutual Kill, which may be a mutual Mercy Kill as well since both of them are already dying.
See also the Fridge Brilliance regarding the theme that plays at the finale, which is titled Happy Ending.
Shocked and upset with what he has heard, Harry secretly comes up with a plan. He asks Brandon to wander around the town with him while reminiscing about their pasts. Harry suddenly tells Brandon that the current successor, Alzac Tino, suddenly falls ill. He casually says that Alzac is such an unlucky guy. Brandon's reaction, however, is only shocked. Not smiling or whatsoever, which may be what Harry is expecting from him if he is still loyal to Harry.
The climax comes at the elevator scene. Harry suddenly persuades Brandon to connive with him to kill Big Daddy, and with this, Harry can reach the top while Brandon will also get Maria back. Brandon is of course, angry because he is loyal to Big Daddy and has left Maria to Big Daddy. He punches Harry and points his gun at him, showing that he'll enforce the Code of Iron out of his loyalty to Big Daddy. However, Brandon then drops his gun, because he can't shoot his best friend.
Unfortunately, Harry perceives this as betrayal and has had enough. He murders Brandon afterwards for betraying him. The next episode, Harry reveals to Big Daddy (who asks Harry to kill him if Harry wants) that he has no intention of killing Big Daddy.Rule of Symbolism). If they team up, they are unbeatable. Their teamwork during their early days at Millennion enables them to rise through ranks very quickly.
- Brains and Brawn / Noble Male, Roguish Male: Harry leads and does business since he has much better communication skills and isn't particularly good at fighting, while Brandon does the fighting and killing since he talks very little and fares much better in fighting.
- Sensitive Guy and Manly Man: Zigzagged. Brandon is an altruistic henchman and cares deeply for the people he loves, but he does all the fighting and killing. Harry is an egoistic leader and does not care as deeply as Brandon does, but he hardly ever gets his hands dirty with guns.
- The Unfettered and The Fettered: Harry does anything to reach the top, even if it means that he has to abandon his morals, while Brandon is limited to his personal code of never betraying anybody.
Rule of Symbolism: Brandon and Harry, Yin and YangThe concept of Yin and Yang is about two contrary forces which are complement to each other. As long as they are balanced, things are fine, but when imbalance happens (can be either Yin excess and Yang deficiency or Yang excess and Yin deficiency), things will go wrong.
In Gungrave, similar thing happens. When Brandon and Harry are still friends and working together, they climb the ladder quickly. But then, Brandon makes a move: declaring to Big Daddy that he will kill anybody who breaks the Code of Iron, no matter who he is. This causes the balance to falter, as the usually passive Brandon suddenly goes against the aggressive Harry. It later ends with Harry murdering Brandon, completely ruining the equilibrium. All these events culminate into the downfall of both Brandon and Harry.
There are also various symbolism of Yin and Yang scattered throughout the anime: the contrast between Brandon and Harry both personality-wise and appearance-wise (dark- and light-colored clothing), the elevator scene in episode 14 (Harry stands in the dark; Brandon stands in the light), the guns position in the final episode when Harry talks about the guns they use during the elevator scene, and finally, the moment before they shoot each other (Harry is covered by the shadows; Brandon is lying under the sun). On another note, the clothing scheme between Brandon and Harry right during the betrayal arc of the anime is a complete opposite. Brandon wears black suit, white undershirt, and black tie, while Harry wears white suit, black undershirt, and white tie.
Another symbolism is Harry is always above Brandon. In episode 4, there's a scene where Harry is sitting on a drum while Brandon is sitting on the floor. Then the moment they decide to shoot each other in the finale, Harry is kneeling while Brandon is lying on the ground. In the flashback that shows the first time they meet, Harry is standing while Brandon is lying on the ground.Earn Your Happy Ending is a trope that appears on both side (which makes the title of the background theme in the finale more ambiguous, because it's called Happy Ending).
If Brandon/Grave dies, he leaves Mika alone, as everybody she loves has gone. On the outside, it's sad. But from the inside, it is a happy ending. In Brandon's point-of-view, his job is done. He has reconciled with Harry, while Mika is no longer targeted by Millennion. Also, he is finally at peace, as being a necrolyzer is miserable; he can decay while being conscious if he does not get the treatment he needs. In this case, Brandon gets his happy ending, while Mika remains as The Woobie, although she appears to have moved on (but still, she is just at most fourteen when all these happen).
If Brandon/Grave survives, Mika will still have somebody to accompany her. Brandon's job is also done: he has reconciled with Harry while Mika is no longer targeted by Millennion. On the outside, it is a happy ending. But from the inside, it's sad. Brandon is not at peace. Firstly, he has lost an arm and a leg (off-screen in the final episode). After that, if he ever lacks the treatment he needs, he will decompose while being conscious. Because the anime focuses on the friendship between Brandon and Harry, Brandon may take it hard when he discovers that in their attempt for a Mutual Kill, he survives while Harry dies. In this case, Mika gets her happy ending, while Brandon remains as The Woobie, although he is 'alive' and presumably continues to be a parental figure to Mika.
As explained by Gary and Widge (to Mika), family is a bond that exists between people as long as they don't betray each other. This is a concept that Big Daddy applies through the Code of Iron, which is punishing traitors with death. Which is why Millennion itself is a huge family (at least, before Harry's reign).
Karma: Laser-Guided Karma, Karmic Death, Karmic Protection, and possibly No Good Deed Goes UnpunishedSurprisingly, karma seems to exist in the anime series. It's especially apparent on Harry and Brandon. In some instances, there is overlapping karma.
Afraid of succession dispute, Harry issues a manhunt on Mika. This forces Mika to run away and causes her to lose her mother and her family butler. However, after Brandon/Grave wakes up, he attacks Harry's Millennion to the point Harry is put into a similar case with Mika. In the last few episodes, Harry becomes the target by the new Millennion and is now the one who is running away. He loses Sherry and the Big Four in process.
Way back, Harry murders Brandon by riddling Brandon with bullets before shooting his left eye. In the finale, Harry is shot multiple times before he is finally shot by Brandon on his head (and in one of the last few shots, it looks like Brandon is actually aiming at the left side of his head).
Throughout the anime, Brandon/Grave doesn't kill any Millennion agents. He only beats them up or in the last episode, shoots them in non-vital areas. He even spares the agents despite having a gun in his hand. Near the end, the Millennion agents only blast him with assault rifles, which barely affect him. Also, the troops with Anti-Necrolyze rifles only shoot off his leg instead of killing him. Then, Biscoe's orders come and the attack is stopped.
And Brandon saying that he can never shoot Harry? There is a very brief scene that shows Brandon fiddling with his old gun after Harry is shot for the first time. Brandon pulls the trigger of his old gun, but there's no gunfire. In other words, the gun is out of bullets (Fridge Brilliance: this resonates with his inability to shoot Harry). Initially, Brandon and Harry are planning to shoot each other with their old guns to 'go home,' but due to the sudden chaos with Millennion agents, Brandon's gun ends up running out of ammo and later, the guns are switched. During their attempt for Mutual Kill, Brandon is using Harry's gun (which he has just used to incapacitate a Millennion agent), while Harry is using Brandon's gun. Brandon is presumably spared for his goodness, but unfortunately, Ambiguous Ending and Brandon finally shooting Harry without taking a bullet from Harry may lead to Fridge Horror (or Fridge Tearjerking), whether he survives or dies due to his Necrolyze wearing off.
The possible Fridge from the said incident interestingly, parallels Mika's loss of Brandon. Brandon has decided to leave Mika because he chooses Harry over her, or rather, Millennion. This saddens Mika very much, who never wants Brandon to leave since she truly loves Brandon and considers him as her family. In the end, Brandon loses Harry by finally (and inadvertently) doing what he never wants to: shooting Harry, without at least taking a bullet from Harry himself. Said inability to shoot Harry stems from Brandon's Undying Loyalty to him.
There's also a moment when Brandon attacks Millennion's research facility, but he does not kill any of the scientists (except Laguna Glock, who ends up killing himself by letting the inferno get him). This enables Biscoe to hire the surviving scientists to produce Anti-Necrolyze bullets, which are later used against Brandon. Due to overlapping karma, he only ends up getting shot by an Anti-Necrolyze bullet on his leg, which cripples him.