Yeah, but it got back to Sora because of Kairi.
Sora WAS a Heartless when he saved Kairi, but Kairi protected his Shadow Heartless form with her love and pure light and brought him back, the first ever as far as anyone can tell.
Sora's Heartless was Pureblood, since it was neither created by Ansem the Wise nor marked with the Heartless symbol.
no way
No. When Sora was turned into a Heartless, Roxas was created as Sora's Nobody. Roxas is the shell Sora left behind when he was transformed into a Heartless, even though Sora later got his heart back. So Sora is not Roxas's Heartless; Sora, to Roxas, is what's known as an Other. An Other is a Nobody's original self. None of the Organization XIII members have an Other who's still alive; Roxas and Sora are a unique case.
If you mean when Sora was a heartless, than it just disappeared. Sora was a heartless, Kairi gives him a hug, Sora hears Kairi's voice and light returned to his heart. He turns from his heartless form to human form, also still making Roxas while turning back into a human.
Sora simply became a shadow. He didn't become an Emblem Heartless, such as an Orcus.
Kairi never became a Heartless. Since she's one of the seven princesses of heart, she is pure of heart and devoid of darkness, so she can't succumb to the darkness and become a Heartless. Naminé was actually born from the transfer of Sora's heart into Kairi. Sora's released heart caused Sora to become a Heartless and produce the Nobody Roxas, but the transfer to Kairi also created the Nobody Naminé.
No. In Kingdom Hearts only Sora became a "Heartless". However, Kairi initially lost her heart on Destiny Islands. Instead of her heart being consumed by darkness and turned into a Heartless, her heart went into Sora's body because of their shared destiny. When Sora found Kairi's body, he released both his heart and her's. Her heart went into her body while his was Consumed by Darkness and he became a heartless. So They did not simultaneously become heartless because by Sora becoming heartless he gave Kairi's heart back to her. And when Kairi lost her heart, Sora still had his.
sora due to his connection with heartless and the seven princesses
Sora did himself. When Kairi's heart left her body, because it had no darkness considering she was a Princess of Heart, instead of becoming a Heartless her heart entered Sora's heart. In order to release her heart Sora stabbed himself with the Keyblade of People's Hearts, a Keyblade created out of the hearts of the Princesses of Heart by Xehanort's Heartless, releasing Kairi's heart back into her body. However, it also released Sora's heart, which became a Heartless, so he did it to himself to save Kairi.
He heart landed in Sora somehow
Story-wise: To effectively defeat more Heartless. Game-wise: To challenge Sora to defeat all 1000 Heartless by himself.
It was just a regular shadow. A bit strange, cause you would expect that someone with a strong heart like Sora would leave behind a very strong heartless.