Although Amiboshi and Suboshi are mirror images in every way except the location and style of their Suzaku marks (on opposite shoulders), the twins could not be more different in terms of personality. It can maybe be said that their extreme experiences helped to develop them both in these diversities, although it seems likely that Amiboshi was always more peacefully minded, and Suboshi always more of a hot-head. Whilst Amiboshi spends most of his time avoiding the fight, Suboshi often charges right in, whether he has the facts or otherwise. However, this is not to say that Amiboshi is incapable of ruthlessness, or that Suboshi is not able to be gentle. The interesting thing about the twins is that they tend to bring things out in one another!
For Suboshi's part, when he believes Amiboshi has been killed by Suzaku's warriors, he jumps to the conclusion that it is Tamahome responsible (likely with Nakago's manipulative suggestion planted in his mind, since Nakago already has a grudge to bear against Tamahome for breaking the Kodoku spell). For this reason, he jumps into the fray, murdering Tamahome's family out of revenge. Even when he discovers his brother is still alive, he doesn't ever seem to show any kind of remorse for the murders of the children and Tamahome's dad, as if he feels that in some way they are justified as sacrifices of war. He even seeks to end Tamahome's life again later when he learns that Tamahome hurt Yui in some way also. (Although it could possibly be seen as a flicker of conscience - do the spirits of Tamahome's dead family come to help him win the fight, or is it Suboshi's guilt which ultimately holds him down and prevents him avoiding his own weapon? The Ryuuseisui are controlled only by Suboshi's own will, so if he had not wanted the weapon to hit him, surely he would have been able to will it to stop. An interesting prospect - did Suboshi seek to end Tamahome's life, or his own, at the end of the day? Had he reached a point where he was sick of fighting, after all?)
Suboshi's trigger-happy temper and obstinate, headstrong decisions get him into no end of trouble. And yet, he is not all comprised of hate and bitterness. Part of his weakness - in fact, the main destructive drive inside of him is not so much that he hates, but that he loves and loves too much. Aside from his devotion to his older brother, for whom he would do anything required, he also develops affection for Yui, and wholeheartedly falls in love with her, sacrificing everything in the end to follow her cause (even though by this time Yui's own heart is beginning to have doubts).
It must not be forgotten that despite his actions, Suboshi is still only the same age as Yui and Miaka and, like they have their first painful steps into love, Suboshi takes his also when Yui shows him sympathy for the apparent loss of Amiboshi. Suboshi's internal drive seems to be to protect those he loves at all costs, even if this means blooding his hands or his weapons in their name. (Whether Amiboshi ever knows how much blood has been shed for him is unclear). Suboshi's loyalty to Amiboshi - regardless of what side Amiboshi is fighting for - overrides everything else, for he even slays his fellow Seiryuu Seishi Tomo to protect his brother - despite the fact that Amiboshi has at this time forsaken the Seiryuu cause to help Miaka and Tamahome. However, it's at times like this that Suboshi is most torn - he saves his brother's life, and seeks to preserve him from the pain of the fight by making him drink the forgetfulness potion - however, within a few moments of that, he's ready to rape Miaka (although whether he really intends to do that or just to hurt her as revenge for Yui is unclear) and/or kill her in the name of the Seiryuu cause.
Although Suboshi has decided to keep following Yui and remain involved with her up to the end of his life, he seeks to protect Amiboshi even then, conveying him back to the Hokkan village where he had been sheltered, and leaving him there to live his life away from the suffering and bloodshed. At this point you wonder if Suboshi already knows that his cause will end in his death, and it's that as much as anything he seeks to protect Amiboshi from. And yet, despite the forgetfulness potion, when Suboshi dies, Amiboshi still sheds tears.
The connection the brothers share runs beyond any normal fraternal bond into one that at times is almost telepathic. Aside from the fact they can send messages to one another by writing on their skin, as soon as Amiboshi falls into the flowing river in Kounan, Suboshi is aware of it. How exactly Amiboshi conceals his life force from his brother between this time and their meeting in Sairou is unclear, but it must've taken some amount of effort to achieve. In contrast to Suboshi's bloodthirsty defence of his brother, Amiboshi does not generally use his power in the series to defend Suboshi. However, it has been suggested and can be assumed that by going to Kounan to impersonate Chiriko, Amiboshi was protecting his brother in another, more subtle way. So long as Suboshi was in such close proximity to Nakago, he was, potentially, in danger...and therefore this is why Amiboshi chooses to jump in the river and possibly drown, rather than betray the Seiryuu cause and give himself up to the Suzaku warriors. He also seeks to prevent Seiryuu's summoning by taking his own life - he sees a bigger picture than his brother and believes that if he can prevent this, he can prevent further war between Kounan and Kutou. However, his reticence to fight does border on avoidance and weakness at times - and it is Miaka of all people who points out to him that sometimes running away can be as counterproductive as facing your demons and fighting for them.
Amiboshi's peaceful disposition only becomes shattered on a few occasions, and he sheds blood unwillingly. He kills a man from Kutou to create his cover with the Suzaku warriors, but admits to regretting it. Despite his tremendous power to manipulate chi through his flute, he does not succeed (or even really try fully) to kill the Suzaku warriors in the shrine (in contrast with the army of Kutou warriors that he slaughters into nothing when they attack the Hokkan village he's come to call home - clearly he can do it properly if he so chooses, so obviously he didn't want to kill those he'd come to respect). He also attacks Tomo with his flute, to break his hallucinogenic hold on Miaka and try and protect her, an act for which he too is attacked and almost killed by Tomo's illusions. Of all of Seiryuu's warriors, Amiboshi is definitely the most reluctant. He is also the only one who apparently never meets Seiryuu no Miko, although he spends some considerable time around Miaka through one thing or another. (Although it isn't manga corroborated, when in the OVA 1 Suboshi possesses Amiboshi's body to rescue Yui from Tenkou, Yui immediately assumes that he is Suboshi, even though Suboshi is dead...probably because she's never met Amiboshi!!).
Suboshi's bond with Yui and Amiboshi's with Miaka are different. Whilst Suboshi loves Yui with all his heart, and while Yui (though never admitting to love) sheds copious tears over Suboshi's death over all of her warriors' passings, Amiboshi and Miaka have more of a sister-brother friendship. Although people have implied that there is romance, I don't really think that there is (and having skimmed through my copy of Ryuusei Den, I think Amiboshi's heart may lie in another direction anyway). Miaka shows Amiboshi kindness, which sparks the gentle side of him, and she also gives him advice and helps him find his courage to get involved enough to save Tamahome and Miaka from Tomo's spell (or at least to try). In return, Amiboshi saves Miaka's life from a wild animal, as well as providing her with time and support as she deals with the fact she may have been raped by Nakago. Although Amiboshi's adoptive parents hint at the idea of Miaka staying to become Amiboshi's wife, neither of them seem to keen on the idea!
My initial impression of the twins was that Amiboshi was weak-willed and Suboshi a homicidal maniac, but I've come to change my mind over paying them more attention. Without the trauma of their background and the influence of Nakago, it seems likely that neither boy would have developed such extremes - Amiboshi is by nature gentle and peaceful, Suboshi hotheaded and impulsive. But I doubt either one of them would have killed, or sought to escape their true selves had it not been for the burning of their village as children. It's not a coincidence, I;m sure, that when Amiboshi introduces himself as Chiriko, he calls himself the sole survivor of a burnt out village. Such a thing is close enough to the truth for him to make it believable, although you wonder if even at that point he had begun to regret his choice, knowing that the suffering of the people in Kounan was similar to the suffering he and his brother shared. Suboshi's hot-headedness makes him much less mature and much less able to think things out properly, which makes him a dangerous, volatile person to be around if you happen to have upset him - as reflected in his violent weapon. As Watase Yuu herself says - he's "too young for war."
Amiboshi is the only Seiryuu warrior to remain alive at the end of the manga. Although he sacrifices his life with Suboshi to save Yui in the OVA 1, this has NO manga basis, and there is no mention of his dying in the latter volumes of the manga. Therefore it can be assumed that Amiboshi survived - although whether or not he continues to live as Kaika and never remembers his brother and his past is somewhat more dubious in my mind. Whether he ever fully forgot the first time is even more doubtful - I suspect that, in time, all of his memories of his true life as Bu Koutaku would have returned, little by little.
Despite Suboshi's violence and occasional lust for blood, then, in the preservation of his brother's life and freedom, he possibly also committed the most gentle, sympathetic, selfless act of all of the Seiryuu warriors...even more so than Soi giving her life for Nakago. In taking Amiboshi back to Hokkan's village, Suboshi is not just saving his life, but also making sure he has a happy, well-protected future. And the fact that he knows this - that he understands what this village has come to mean to his brother - suggests a level of understanding between the twins that is unspoken. In the end, though he continues to fight for Yui, it's probably true to say that the one Suboshi truly loves most of all is his older brother.