Basically Tsuna has a different set of elements. But not normal elements, each of Tsuna's elements is also a Sky (or an Earth). Image Reborn and Vongola reaction to this situation XD

Cloud - Kyoko

Sasagawa Kyoko is a welcomed Sky. Her first harmonized element was Cloud. It's ironic cause she's also a Cloud as Tsuna's first harmonized element and she also only had a Cloud as an element when she met Tsuna. Her Cloud was Kurokawa Hana. Her other elements are her brother - Sasagawa Ryohei as Sun, Miura Haru as Lightning, Yamamoto Takeshi as Rain, Nagi or Dokuro Chrome (and Rokudo Mukuro tag along) as Mist. Kyoko doesn't have a full element set, she doesn't have a Storm.

When Kyoko and Tsuna met, their flames just clicked. Kyoko was shocked cause it wasn't the same as when she and Hana harmonized. She wasn't the Sky welcome the other elements. This time, it was her who was welcomed to another Sky. It was the first time her Cloud flame show up. She knew for sure that this boy is her and she protects her properties.

Later she explained it to Hana and Hana told her that the same feeling she has toward Kyoko.

Sun - Enma

Kozato Enma is a strong Earth. He already had half of a full element set before he met Tsuna.

When Enma and Tsuna met, their flames didn't react much. But after a few meetings and suddenly they bonded. Enma knew that they were bonded but he just confused cause the bond between him and Tsuna different from the bonds he has with his other elements. But Enma knew that different or not, from that moment, Tsuna is his and he is Tsuna's.

Later when he got his full set of elements, he realized that he isn't an Earth but a Sun to Tsuna.

Storm - Xanxus

Xanxus di Vongola is a powerful Sky. His Storm flame blend in his Sky flame and created the Flame of Wrath. He already had most of his elements except for a Mist and a Cloud before he met Tsuna.

When Xanxus and Tsuna met, let just say there were a lot of flames happened. In the end, Xanxus had to give his new Sky a lesson about flames and harmonized elements. When Xanxus being introduced to his Sky's other elements, to say that he not surprised is a lie, Xanxus knew that his Sky is special (cause the fact that Tsuna can make him, make Xanxus become his element say something) but he not expected Tsuna's other elements are same as him. It's also a delight to be introduced to his going-to-be-Cloud, a certain Hibari in Namimori.

Later everyone in mafia world confused when Xanxus never seeks for a Cloud.

Mist - Byakuran

Byakuran Gesso is a chaotic Sky. He has a full set of elements, some of his element positions are taken by more than one person. He had most of full element set before he met Tsuna.

When Byakuran and Tsuna met, chaotic endured and their flames just clicked much to everyone's horrors. Byakuran was sad that he isn't his Tsu-chan first element. But then Tsuna introduced him to his going-to-be-second-Sun, Irie Shouichi, he was very delightful.

Later everyone confused when the Gesso heir got along with the Giglio Nero heir too quick.

Lighting - Dino

Dino Cavallone is a strong Sky. He didn't have any other elements except Romario as his Lightning when he met Tsuna. It's another irony thing cause Dino is Tsuna's Lightning.

When Dino and Tsuna met and Tsuna call him Dino-nii, something in Dino just sparked and reached out and then Dino realized that he is bonded with another Sky. Dino was sad when they had to stay apart but Tsuna promised to keep in touch with him. Image how horrified Dino was when he met Xanxus after his bonding with Tsuna. He can fucking feel the bond between Xanxus and his Sky which means Xanxus also is Tsuna's harmonized element. They had a staring contest until Squalo broke the weird silence between his boss and the Cavallone heir.

Later when Dino met Tsuna again, Tsuna unintendedly gave him a Storm as he introduced Gokudera Hayato to him.

Rain - Uni

Uni Giglio Nero is a wise Sky despite her age. She doesn't have many harmonized elements yet. It was a surprise that despite being the Sky Arcobaleno, she also is a Rain.

When Uni and Tsuna met, there was a mutual understanding between the two of them that no one understand and their flames just dancing around each other.

Later there also are some weird understandings between some Sky heirs that leave everyone else in the mafia world confused.

Reborn

1. Reborn always aware of the weird interaction between the Vongola Nono's son, the Varia boss and his student, Pipsqueak Dino or the weird friendship between his niece, the current Sky Arcobaleno, Uni and that weird Gesso heir. But he never really care enough to investigate. He really regretted that after he met his second student.

2. Reborn expected a no-good teenager with no bonded elements. But instead, he got a powerful and full harmonized Sky. He burned the little respect he have for the CEDEF head and the file that Iemitsu gave him after that moment.

3. When Reborn first met Tsuna's element, he was caught off guard. Because Sasagawa Kyoko is not only a Cloud but also a Sky with a almost full element set. The fact that Tsuna had a full set of elements didn't surprise Reborn cause he can tell that Tsuna is a powerful Sky. But his element also is a Sky, not very much.

4. Reborn can tell that something really wrong with the way Pipsqueak Dino stunned when he show him Tsuna's house. He confirmed his instinct was right when Tsuna showed up and huged his former student and called him "Dino-nii". Reborn really wanted to shot a certain CEDEF boss right that moment.

5. Reborn should have expect this with the weird interaction between his former student and Xanxus. But seriously, another Sky as element? He did shot Baka-mitsu after that.

6. The ring battle didn't happened. After a staring contest between Tsuna and his Storm, Xanxus didn't give a fuss about Tsuna being the Vongola Decimo and released a confused Vongola Nono.

7. When Uni told Reborn that she was going to visit him, he didn't really think much. He regretted it the moment his niece and the Gesso heir bust into his student's room and hug him and their flames just dancing around each other and he realized that Uni is a Rain and both of them are his student's elements. Reborn thought that he should get a vacation.

8. The moment the Simon Decimo show up, Reborn ready to get a vacation. He would leave the whole mess for Vongola to deal with. But he really impressed with his student's element set. A Sky with Skies as elements.