Reborn is Timoteo's most trusted assassin. Timoteo is Vongola IX and as such, is the one who asked him to teach Tsuna how to succeed the Vongola Famiglia. Reborn thinks very highly of him and respects him. They often communicate with letters about Tsuna's progress and growth. The Ninth seems to reveal many secrets of the Vongola to Reborn since he knows that Reborn can be trusted with such information.
When Xanxus stated that he had done something to the Ninth at Lightning Ring Battle, Reborn was already prepared with his gun. Also, when the Ninth was revealed to be inside Gola Moska as part of Xanxus' scheme, Reborn stated that he had already lost his patience but wouldn't fight out of respect for the Ninth's will. He also often convinced Tsuna to think about the Ninth's feelings when Tsuna refused to take part in the Inheritance Ceremony.
I've lived so I don't have any regrets whenever I die... But now I want to see you grow a little more... So, let me live, Tsuna. [...] I want to live longer.
Tsuna is Reborn's student. Reborn was sent to train him to make him a true successor of the Vongola Famiglia. He is quite hard and fierce on Tsuna, using Spartan training methods. Which often involves Tsuna getting shot, facing danger, and humiliation. This and due to Reborn's involvement with the Mafia, makes Tsuna wary of him.
However, they later form a bond of friendship that allows them to work together after the many situations they have been through together and this bond is what makes a drastic improvement in Tsuna's ability and character to lead the Vongola, something that Tsuna recognizes, though Reborn still largely remains a teaching figure. Reborn has also shown concern for Tsuna's well-being, as shown when he became frustrated at seeing Tsuna beaten up by Demon and wanted to help him if only the Vindice didn't interfere.
Colonnello has said that Reborn is like another father for Tsuna, as he is always there for Tsuna when he needs him and will do anything if it will help Tsuna grow. The extent of Tsuna's and Reborn's bond is shown to be very deep, proven when Tsuna shouts at Reborn when the latter about to give up the Representative Battle, which finally bring up Reborn's desire to live longer in order to watch his student's growth.
Dino was Reborn's student before Tsuna. Reborn was sent to train him to make him the Boss of Cavallone Famiglia. Similar to Tsuna, Dino was treated fiercely by Reborn with Reborn's Spartan methods of training that made the young Dino become wary of him, and until now, still does. Reborn often likes to tease and make fun of Dino, referring him as No-Good student of his along with Tsuna.
Despite Reborn's treatments, Dino has come to respect Reborn and is grateful to him since it is thanks to Reborn's training that Dino has become a great and dependable Boss for his Famiglia. Dino and Reborn care for each other but are not quite as close as Tsuna and Reborn are.
Reborn briefly tutored Yamamoto during the Future Arc. Although Yamamoto sees Reborn as a child, he respects the latter's power and skills. Reborn also favors Yamamoto and even revealed an Arcobaleno secret to him after Yamamoto was able to land a hit on Reborn's hat in combat.
Reborn forms a special relationship with Luce, Aria, and Uni, who all share the same blood and destiny. Before becoming an Arcobaleno, Reborn seems to be close to Luce and cares for her. Luce was also the one who commented that Reborn's sideburns were his most attractive feature. Reborn is mostly seen with Luce and they were shown to have talked about personal or Famiglia problems.
After Luce's disappearance, Reborn was also the first person who realized the resemblance of Aria with Luce and apologized to her for forcing her to carry the Arcobaleno curse. Aria also sees him as a good friend possibly because of his relationship with her mother.
In the Future, the first time Reborn met Uni, he immediately recognized her, just like when he met Aria. Uni also immediately called Reborn "Uncle." He can act very protective of Uni because she is the Boss of the Arcobaleno. He is also the only one who understood her feelings and her destiny as the Sky Arcobaleno, just like her mother and grandmother.
Colonnello, Lal, and Fon are Reborn's old friends. Colonnello claims to be his great rival but they actually have a lot in common and share some of the same qualities and traits, like having a sharp tongue and sleeping out of the blue. They also often punish Skull when he had done something stupid.
Lal acknowledges Reborn's strength and even admitted that he was the strongest Arcobaleno, though she is often irritated by him because he knows what she was thinking and what action she will take, which ends up in argument. Reborn sometimes like to tease Lal since she doesn't have a sense of humor.
Reborn rarely interacts with Fon since Fon lives in China, but they are on good terms. Reborn acknowledges Fon as a high-skilled martial artist and both of them also give some suggestions to each other whenever needed.
Skull is only seen as an errand boy or a lackey, which annoys Skull because he feels that Reborn never sees him as an equal, but also found that he's afraid of Reborn when he starts to punish him. Skull often tries, to no avail, to prove that he is superior that always resulted in him being punished by Reborn.
Reborn and Viper don't seem to be on good or bad terms; however, Viper thinks of Reborn as a shameless and foolish person but, at the same time, is also aware of his strength.
Out of all Arcobaleno, Verde is the one that Reborn doesn't get along the most. Reborn never considered Verde as a comrade and found it hard to trust him due to Verde's obsession with his research and the fact that he will do anything to accomplish it. Verde only thinks of his research and didn't care when he broke the Arcobaleno Pact, and he sees Reborn and the others as merely a subject of his research. He commented that he found it unpleasant to be with the other Arcobaleno and Vongola. Their mutual dislike towards one another is furthered shown when Verde joined the Representative Battle simply because he wanted to defeat Reborn.