His work

...as tutor...and hitman

Reborn is well known hitman in the underground world, and the inteligent people fear him, and try not to cross the roads with him. Besides the fact that he sacrificed himself for the ultimate power, giving up his adult body, he also is one of the most trusted members of Vongola family, and because of that Vongola Nono gives him the job to go to Japan and train Tsuna to become worth the title of the Vongola Decimo.

Besides training his student, he took upon himself the job to select the six guardians for his student, the ones that will be with him in the future and that they are worth of trust and also enough powerful to be able to defend their boss, who still should be stronger and more authorative then them.