The most striking resemblance of Xanxus and Ricardo (pictured to the left) is their messy black hair and their sky flames of wrath. They also seem to share their tempers and violent personalities. This particular flame is very rare and had not been seen since the time of the Second Boss's reign. For that reason, Xanxus' mother believed that he was very much the son of the Ninth Boss, Timoteo. She convinced herself and her son of this and Xanxus was ultimately taken into the Vongola Household.
Xanxus' ability to use the flame of wrath is an extremely important part of his character. It is one of his most defining traits and people fear and respect him for it. It is a convenient way to include him in the story and it parallels the First and Second boss's rivalry as his flame puts him against the first boss's mirror tenth candidate. According to Xanxus, the flame of wrath is far more powerful than the pure sky flame of the First or Sawada, this power is the reason that the First Boss fled to Japan. Xanxus uses this as fuel to test his power against the last tenth boss candidate.
In addition, Xanxus also uses the ability of putting his sky flame into his pistols to increase their power much like the Seventh Vongola Boss, Fabio. Initially, the dying will gun was made to channel and enhance the flame of the user. Fabio was considered to have the weakest flame of all the bosses and thus created the gun to assist in strengthening his flame. Xanxus uses a modified version in two pistols that allow him to channel his already powerful flame into an even more destructive force. Fabio was the father of the Eighth boss and thus the grandfather of the Ninth boss which makes him the Great-grandfather of three of the five Vongola Tenth Candidates. Archives for weapons used by previous bosses would be easy access to any Vongola Boss candidate. As such, it makes sense that Xanxus would come to know and understand the usage of the dying will guns, that he later takes and uses himself.