There aren't my theories, but a combo of a couple of friends' theories.
Kairi is a Princess of Heart, thus when her heart was released, a Heartless was not formed, but another form of some sort, perhaps a spirit, which Namine supposedly is. Xion resulted as the Nobody.
Xemnas began to become Xehanort-obsessed, and as a result, no longer recognized Xion as a member of the Organization and killed her, provoking Roxas, who held her in high regards, to leave the Organization because he cared about her a lot. (A love triangle of sorts, involving Riku, Xion, and Roxas. Xion liked Riku and Roxas cared about Xion.)
The Oathkeeper keyblade was a symbol of Kairi's promise to Sora, and was also a symbol of Roxas and Xion's friendship, saying that when they found their hearts, they'd love each other. It also originally belonged to Xion before she was killed.
The Oblivion keyblade represented Riku. In a way, Roxas would have Riku and Kairi by his side as his source of power.
When Xion was dying, she gave Roxas Oathkeeper to remember her by, and his 'anger' was provoked. As a result, he swore revenge on Xemnas, but was nearly killed by the Superior if not for Saix's intervention to stop Xemnas.
Marluxia was nearly killed as well by Xemnas, but if not for Roxas protecting him, exclaiming "Why are you doing this?"
These aren't my theories, so don't blame me.
Kairi is a Princess of Heart, thus when her heart was released, a Heartless was not formed, but another form of some sort, perhaps a spirit, which Namine supposedly is. Xion resulted as the Nobody.
Xemnas began to become Xehanort-obsessed, and as a result, no longer recognized Xion as a member of the Organization and killed her, provoking Roxas, who held her in high regards, to leave the Organization because he cared about her a lot. (A love triangle of sorts, involving Riku, Xion, and Roxas. Xion liked Riku and Roxas cared about Xion.)
The Oathkeeper keyblade was a symbol of Kairi's promise to Sora, and was also a symbol of Roxas and Xion's friendship, saying that when they found their hearts, they'd love each other. It also originally belonged to Xion before she was killed.
The Oblivion keyblade represented Riku. In a way, Roxas would have Riku and Kairi by his side as his source of power.
When Xion was dying, she gave Roxas Oathkeeper to remember her by, and his 'anger' was provoked. As a result, he swore revenge on Xemnas, but was nearly killed by the Superior if not for Saix's intervention to stop Xemnas.
Marluxia was nearly killed as well by Xemnas, but if not for Roxas protecting him, exclaiming "Why are you doing this?"
These aren't my theories, so don't blame me.