The Sakurazukamori is always alone.
From: Tokyo Babylon & X/1999 (CLAMP)
A master of the duplicitous arts, Seishirou has lived his entire life within a self-created world of deception. By right of birth, he holds the honor of protecting the fate of Japan, his home country, using onmyou-jitsu (a form of Japanese spellcasting). People who threaten Japan's well-being are generally killed, a task he has a special aptitude and affinity for. He confidently ensnares his prey with cleverly crafted illusions, marked by the blossoms that share his name (sakura, Japanese for "cherry"). Because of his higher calling, it is imperative that he not form any emotional attachments, since thoughts of love and family would interfere with his mission.
Such a lonesome lifestyle might break a weaker person, but the emotional weight of being the Sakurazukamori appears not to bother Seishirou very much. Usually cold and often heartless, he insists to anyone who asks (and a few that don't) that he is wholly incapable of feeling fondness or compassion. He even takes the assertion so far as to make a very special bet with a very ordinary little boy that would determine once and for all if the Sakurazukamori could be anything other than alone.
The arrangement was thus: If, when the boy was older, Seishirou could bring himself to care a whit for him, then the boy would be free from the deadly attention of the Sakurazukamori forever. Otherwise, if the days of wine and honey had passed and our assassin's cold heart was still free from love's light, then the younger boy's life would be forfeit. The twisted tryst was sealed not with a kiss, but with branded palms, marking young Subaru as the chosen prey of the Sakurazukamori.
Of course, it would seem that the kid has more to lose by the bet than Seishirou, but an admission of feelings, for anyone or anything, would be a sort of failure, since part of 'Sakurazukamori' means 'one'. Seishirou spends a good deal of time and effort under the guise of 'Sei-chan', a happy homosexual veterinarian in love, in order to push the limits of his supposedly uncaring nature. Seemingly at odds to his insistence on an emotionless existence, Seishirou willingly subjects himself to a very difficult sacrifice to protect the object of his non-affection. (Lost an eye for some guy? No passion there, I assure you.)
Nonetheless, when the day of reckoning arrives, Seishirou is confronted with his own ill-considered vow and has the kind of reaction many gamblers have on 'the morning after'. If he admits, even to himself, that one person has become special to him, then the lie is given to everything that he's built his life upon. Add to that the further wrinkle that the one who is loved by the Sakurazukamori must become the next Sakurazukamori through a particularly bloody death-rite (it's in the contract, I swear).
Naturally, faced with such a choice, our boy can only take one route...the kid is toast. But when love enters the equation, everything goes up in smoke (or sakura petals, as may be). Instead of a nice, clean death which could bring neat closure to the whole dilemna of emotions, Seishirou fails (purposely?) to kill the puppy-eyed, smooth-faced lad. Now a horribly protracted game of cat-and-mouse unfolds, in which both participants pick up sidekicks and unhealthy characteristics of the other.
Without closure, Seishirou is left in emotional limbo, and he still doesn't even admit that he has emotions. Fulfilling his destined role as one of seven Dragons of Earth provides many opportunities to taunt and torment his former foundling, but after a while even that game loses its sparkle. Ultimately, however, games of youth must be put aside. Seishirou's gift and curse was to always know when that time had come. Perhaps because illusions were his stock and trade, stepping out of his own deception was his hardest challenge and greatest achievement.