Lavina is Hayato's biological mother. Due to the mafia law, Lavina was only allowed to see Hayato three times in a year without revealing her true identity to him, since she was a mistress. Regardless, Hayato grew to like Lavina after bonding with her through piano and through her encouragement Hayato dilligently improve his skill as pianist. Lavina loved her son very much and always looked forward the times she's allowed to meet him until her death. Hayato was very saddened by her death, especially after knowing that she's his real mother. Eventually, he ran away from home and threw away his skill as pianist out of anger at his father, though he can't completely let go of it as it still reminds him of his mother.
Bianchi is Hayato's half-sister; as such, they have brother-sister relationship. Bianchi revealed that their common parent is their father and Hayato is an illegitimate child. Hayato cannot look at her without being sick due to the food poisoning he suffered from Bianchi's cooking when they were little. Bianchi later remedies this by covering part of her face when she is near him, like goggles. Their strained relationship stems from their youth, when Bianchi would use Hayato as a test subject for her latest poisonous creations. Despite that, Bianchi cares a lot for Hayato, as indicated when Hayato gets severely injured after a battle with Chikusa, she displays anger and worry, wanting to take care of him as an older sister. She's also very happy when Hayato implied that he worries for her.
After being saved by Tsuna, Hayato pledged his loyalty to Tsuna as the Vongola Decimo Boss of the Vongola Famiglia. He is fiercely loyal to him going to extremes to prove his worth. As before he met Tsuna, he was always alone and never being able to trust anyone, and as such is grateful that Tsuna changed his life, that he is not alone. He also claims to be Tsuna's right hand man even though Tsuna thinks mostly of him as a friend. (In the future, he does become Tsuna's right hand man. But since they returned in their time period, they changed the timeline so it's no longer sure that he'll really be Tsuna's right hand man latter) During his fight with Belphegor, he was willing to die in order to gain the Vongola Storm Ring but was persuaded by a desperately pleading Tsuna to give it up so he could return safely without dying. While in the future, Hayato disobeyed an order from Tsuna for the first time in order to help bring the family back to a peaceful past.
Hayato sees Takeshi as a rival for the position of Tsuna's right-hand man and as such seems to dislike him intensely. He is sometimes very jealous of him of either his talent or his close relationship with Tsuna. Hayato also constantly makes fun of his love of baseball. While the two were in the future, Takeshi told Hayato that he was unworthy of becoming Tsuna's right hand man as he turned away from his fellow Guardians instead of uniting them. This prompted Hayato to finally work together with Takeshi. Despite this, Hayato seems to care for Takeshi, as he was shown to be angry that Takeshi got injured by Kaoru during the Inheritance Ceremony Arc.
Hayato met the doctor when he was young and was fascinated by his use of dynamite. He wanted to be his apprentice but was turned down when he wounded himself in a fight. He was later accepted by Shamal who trained him to fight the Varia as long as he didn't fight recklessly. Hayato is often annoyed by the doctor's riddles and sees him as a perverted old man. Despite this, Shamal is the only home tutor compatible with Hayato.
Lambo and Hayato have a strained relationship since Hayato sees Lambo as pretty much an annoying little kid who causes nothing but trouble. Despite this, however, Hayato seems to care deeply for Lambo as evidence of him saving him from Levi A Than's attack during the Varia Arc. At the same time, Reborn also mentions that the Storm and Lightning have somewhat of a brotherly relationship, even if Hayato treats Lambo like an annoying younger brother.
Hayato took an interest in her hobby and referred to her as an U.M.A. (Unidentified Mysterious Animal). Hayato has problems calling her the name she wants to be called (the only name she would respond to): Shittopi-chan. His interest about her ways turned into dislike when he realized she is not an U.M.A, more so when she became his stalker after she became interested in him. His dislike of her is so intense that he would rather avoid her than be around Tsuna.
At first, Hayato showed somewhat antagonistic treatment towards G., claiming that he would get his inheritance and surpass him. However, ever since he gained his inheritance, Hayato has been shown to honor G and look up to him, once even saying that he wished to follow in his footsteps.