Xanxus' first role is as the second main antagonist of the series. He serves the purpose of pushing Tsuna to his limits to create the abilities that Tsuna needs to be a good boss. Xanxus and Tsuna are polar opposites, their base desires are broken down into rage and hate and love and calmness. They are everything good and bad about the other. Xanxus, eight years after his defeat at his father's hands, collects the Vongola Rings and challenges the last candidate for the Tenth Boss position to an official battle. The ring battle serves to appoint the best suited members to the ruling of the tenth generation Vongola family. Xanxus and the Varia enter this battle against Tsuna and his friends. While an official battle for the family and sanctioned by the mafia world, it's really just an excuse to kill Tsuna and his father. After Xanxus' defeat he still refuses to accept Tsuna as boss, but supports the family to the fullest.
His second role is that of a supporter to the cause of standing against a rival family, the Millefiore, in a story arc that takes place ten years in the future. In this time, Sawada has become the boss of the Vongola and a thirty-four year old Xanxus still refuses to accept it. His men go so far as to create a banner that states "we do not support the Vongola Tenth" just to make Xanxus happy. On the other hand, the varia take over an enemy stronghold and it is destroyed despite their best efforts, Xanxus defends what's left of it. Despite his ideology not mixing with Tsuna's he believes with every fiber of his being that they may have internal conflict but when an problem arises from the outside that the Vongola stand together. He wants the family at its strongest. He understands that Sawada delivers this. For that reason he will defend the family with everything he is. He will bend to the will of Sawada insofar as keeping the family standing and strong. Yet, for the base of desire, does not and will not openly support its boss. After everything is sorted there, everyone goes back to the present time to deal with an inheritance ceremony.
Xanxus refuses to attend the Inheritance ceremony because Sawada Tsunayoshi is present. Instead, he sends his men to represent the Varia even though as the boss he should be there.
During the final arc of the manga, The Representative of the Rainbow Battle, Xanxus serves on Team Mammon as the boss. He accepts any chance to smash in the Tenth Vongola's face. He enters as an antagonist to the Rainbow battle for the Arcobaleno, Mammon. He becomes an unwilling supporter when the tides of battle change and the Vongola is in trouble again. He supports Tsuna's plan, as a member of the Allied Team after his own boss watch is broken, only because it benefits the family as a whole. His acceptance of role had more to do with someone being more irritating than Tsuna too, in which Xanxus suffers great injuries from his involvement but is contented to be included in the most elite force of the battle.
Ideally, he would serve as a very good counter to Tsuna's compassion and idealism. Xanxus is far more the realist and willing to take chances and I feel that if the two could ever reach common ground they'd be unstoppable. Yet, Xanxus remains stubborn and proud. Tsuna, for his part knows that ultimately he can rely on Xanxus' strength; better yet, he knows better than to push the limits without a very good explanation.