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When Michelangelo saw him for the first time after thirty years, he felt resentment.
Even when he finally got close after defeating the police force and upon seeing that this Donatello was still in his teens, the resentment didn't ease.
However, the longer he remained in this Donatello's presence, the more the resentment faded and the one light that Michelangelo had crushed a long time ago, began to glow once more.
That light, hope, filled his chest and he began to wonder, as he listened to his alternate dimension brother speak, if this was truly the beginning of the Shredder's end.
As Donatello berated Raphael and Leonardo, Michelangelo knew already that no matter what they said, his brothers would join him in aiding Donatello in ridding the Shredder once and for all.
And even when they were in the Shredder's palace, fighting Karai Legion Bots that had surrounded him; even as he called out to his brother to apologize for everything, the hope that had formed in his chest didn't dim.
Even as he fell, defeated for the final time, hearing his alternate dimension brother's anguished cries, the hope grew, even as his spirit left to join his father.
He didn't regret dying this day, knowing that this was the day that the Shredder would truly be defeated.
Perhaps… when he got to the same sky that his father and Casey resided in… his Donatello would already be there.
And if he wasn't… then he would search the earth while within the sky to try and find him, to watch over him.
He spent too long harboring a grudge, resentment, towards his older brother.
It was time to let it go.