When Haru first met Tsuna, she considered him a threat who she greatly disliked, under the impression that he had corrupt the mind of Reborn with the mafia, her hatred towards Tsuna extended to the point of her physically harming him. After Tsuna rescues Haru from drowning while in Dying Will Mode, she develops an admiration towards him, and makes it her goal to become his wife sometime in the future. From this point on, Haru is generally more friendly towards Tsuna, only scolding him on occasions where she believes that he's mistreating Lambo or I-Pin.
Their friendship gets somewhat strained while in the future, where Tsuna is accused of "not knowing anything" by Haru, after he yells at Lambo for bothering him and getting angry at Haru for not looking after him, Haru, in tears, states that she was willing to listen to Tsuna, but he never tried explaining anything to her. Tsuna later apologizes to Haru, after realizing how much she and Kyoko have helped them while in the future, and the two rekindle their friendship. Only a little while later, the two have another falling out, with Tsuna refusing to explain the mafia and Millefiore towards Haru and Kyoko, which makes Haru feel left out and concerned, Haru joined Kyoko in going on strike, refusing to cook and clean, until they are informed about the mafia.
Haru is often prone to get angry whenever Tsuna interacts with other girls, this can be shown whenever Tsuna blushes at Uni's smiling much to her anger, and gets angry at Chrome Dokuro for kissing Tsuna's cheek as a form of greeting in the non-canon Haru's Haru Haru Interview: Dangerous.
Since the two first met, had become the closest of friends, as the two both shared a love for eating cake and created appreciation days for their love for eating cake, which they coincidentally have on the third sunday of every month. It's been stated that both Haru and Kyoko are in sync, to the point that they both share the same Dying Will regret, though they also have differences in personality, as Haru is shown to be more energetic and assertive.The two have been shown to bond with each other further in the series, as they become close enough to have sleep overs with each other, and attend many events together with Tsuna and his friends.
Together, they both support Tsuna and his friends on the sidelines, providing them encouragement and act as a driving force for both to continue fighting. While trapped in the future, Haru helps Kyoko in helping clean, cook, and look after the children, though they gradually begin to feel betrayed and left out whenever Tsuna refuses to explain the mafia or Millefiore, though recoil their friendship with Tsuna after he finally explains the mafia to both of them.
Since the two first met, Haru had instantly took a liking towards Lambo, due to her love towards children. As opposed to others, Haru isn't bothered by many of Lambo's childish antics, and becomes very protective over him, especially whenever he is bullied by Hayato. Haru is shown to care for Lambo, as she is always seen taking care of Lambo in the absence of Nana Sawada and Tsuna.
Despite her adoration to Lambo, she greatly dislikes the ten year old counterpart of Lambo, who she considers perverted due to his revealing outfit, and often physically harms him because of it. Haru is unaware that fifteen year of Lambo is the same person as his five year old counterpart.
When Bianchi and Haru first met, they shared a common interest together, both hating Tsuna and loving Reborn. It was because of Bianchi that Haru learned about Tsuna's role in the mafia and the Reborn was a real hitman. Bianchi is like an older sister figure towards Haru, helping her with cooking and her admiration towards Tsuna. Haru's relationship with the future counterpart of Bianchi seems to remain the same, with future Bianchi comforting Haru on different occasions while trapped in the future.