Warning. If you would like to watch ROTTMNT which I hope you do these contain spoilers to the show and movie.
This also contains angst and stuff so if you don't like sad boo-hoos you may wanna leave. This is kinda y first time making soemthing really angsty sooo it may be bad- anyways enjoy

It's been three months. Three months since the Kraang invasion. Three months. Three months of Leo trapped in the prison dimension... and killed before Mikey could save Leo. It hurt so much for Donnie, as much as he hated his brother he loved him dearly, and now...he's gone. No one was taking it easily, Casey Jones Jr seems to be the embodiment of Leo in fact, no one blames him since he was raised and trained from a future Leo. But either ways the pain hurts, Donnie would spend hours working on new inventions maybe even ones where he can turn back time, but nothing. All he received was coffee and being a sleep deprived teenager.
He could've done something, just anything but all he did was stand by and watch, if he had done anything maybe even used the mystic drill he could've lived, but no. Donnie choked that moment, he froze, before he could do anything Leo was stabbed. His last words hurt him even more like a something had cut him deep.

"I love you Hermanos, and nothing can change that, just so you know, this isn't your fault.."
Seeing the battered and bleeding Leo, just drifting in the prison dimension, bleeding, his shell fractured. But Donnie was extremely upset, how could he say it's none of their fault?! It was clearly his! Right..? He wasn't sure it was a question he ponders often, even when everyone reassures him he just snaps at them, he's became relevantly more isolated. Even if Mikey tried to cheer him up with pastries or songs, or when Raph gives him hugs, even when they mention bringing him uranium from Barron Draxum.
Nothing. Not April's fun surprises, not Splinters wise words, nothing. He can't help but always feel angry when he sees Casey. It's wrong since he's also just a kid but Casey was just too much of a parallel to Leo, he hated it. He never let Casey talk to him, never let him be near him, and certainly didn't let him in his room. Raph was elected leader again, but luckily the crime rates have gone down since the attack. Donnie is sipping on a cup of Lemonade made by Todd, Todd decided to visit maybe cheer Donnie up, despite Donnie still being a little uhh sour. Then his computer beeps.

"Oh great criminal activity."
Donnie says with an unenthusiastic tone before he grabs his Bo-staff and sarcastically says.

"Hello stupid stick time for some action."
As he spins his Bo-staff as a quick warm up, Mikey rushes into his Donnie's room at that moment.

"Donnie was that criminal activity on your thing?"
Mikey asks he seems prepared to jolt out, Donnie sighs rubbing his eyes before replying to Mikey.

"Yes. Mikey."
He says slowly and a bit dramatically.

"Okay then let's go!"
Raph says as he punches his hand he's clearly ready for a fight.

"Can I come too?"
Casey asks nervously to Raph.

"Of cour-"

"No."
Donnie interrupts Raph.

"No? Why no? Donnie it's better if it's four people, this looks like a four person job by the looks on your tech thing."
Raph crosses his arms sternly at Donnie, his computer screen was beeping red which means the enemy is strong but it wasn't pink.

"It's not the end of the world situation it's not blinking pink."
Donnie huffs.

"Donnie you can't just keep pushing us away!"
Raph raises his voice a little his voice filled with concern.

"Wha- I'm not pushing anyone away."
Donnie says his eyes widening with shock, he's clearly offended.

"But you are Donnie."
Mikey admits.

"Wha- fine. Fine. The embodiment of Leo can come."
He sighs, as he spins his bo-staff once more and starts heading out, leaving Raph, Mikey, and Casey concerned as they follow behind.
Everyone makes their way up on top of the NYC building, there was nothing out of the ordinary though, cars were beeping loudly though,
Someone is struggling to pass them, they looked panicked.

"That's the crook!"
Raph says pointing at the very tiny person, which made sense since they were far away.

"Yeah no kid Sherlock."
Donnie replies sarcastically, everyone looks at Donnie with a frown.

"What? I'm just saying it's pretty obvious and-"

"Donnie. Don't use your sarcasm right now and listen to the plan."
Raph grumbles.

"Fine fine."
Donnie raises his both his hands near his chest clearly annoyed.

"So what we're going to do is check out this rookie from afar, once we identify them we can regroup and come up with another plan."
Raph says, everyone nods but Donnie who is stubbornly shaking his head.

"That sounds stupid. I have a better idea let me do the work."
Before anyone can argue, Donnie turns up the music on his head phones, he listens to 'one day by Lovejoy'  then charges right in at the person, as he starts to get closer he can see the person better, he's...he's a big turtle mutant, he's an adult, he's wearing a fake left arm, he has yellow stripes, and two red stripes close to his blue mask like a clowns...

"Leo?"
Donnie realizes this looks like Leo, but his Bo-staff was already aiming at this Leo-looking mutant.

"Donnie?"
The adult turtle mutant says back his eyes wide with shock, then Donnie hits him on the head with his Bo-staff, knocking him out.

"Wha-?"
Donnie gasps as he puts it away as he the Bo-staff, the cars pass them quickly, as Raph, Mikey, and Casey stands next to him.

"Look what you did Donnie! You knocked out a criminal."
Raph grumbles rubbing his temples.

"Well at least he's knocked out we could just turn him into the cops."
Mikey says trying to stay optimistic.

"Sensi?"
Casey says seeing this Leo-like mutant.

Everyone just stared at this old turtle mutant, he looked just like the Leo they know but more wrinkly and big.

"Wha the-"
Raph says in shock, Casey starts to carry this older looking Leo and Raph helps him.

"We are taking him back."
Casey says.

"What no! That thing can- that thing can be kraang!"
Donnie tries to protest.

"Sorry Donnie."
Mikey sighs as he also helps, anger rises in Donnie as he reluctantly follows.

As the first action of getting him they made him sit down to relax, but since Donnie is so stubborn he ties him up, the knot was a complex knot called the Constrictor knot. He seems proud of himself from a single knot that no one agreed on using.

"I'll wake him up with my taser."
Donnie says a little maniacally.

"No! Well interrogate him when he's awake-!"
Mikey says blocking Donnie from using the taser.

"Cant we just untie him-?"
Casey seems to not like this idea at least one bit.

"No! Casey you've been nothing but trouble."
Donnie grumbles stubbornly.

Casey seems hurt at this comment, his lips were quivering s little. He clenched his fists.
"Okay- I understand that you miss Leo but that doesn't mean you have to target me!"
Casey raises his voice a little clearly hurt. Donnie seems to have no remorse and his brothers just shake their heads at him with a frown.

"Donnie what's up with you even!?"
Raph groans.

"I'm just saying- this guy is a literal Doppelgänger! Both of them!"
Donnie points his finger at the elder turtle tied up to the chair and Casey.

"What?! Donnie you can't say that!"
Mikey gasps clearly starting to get fed up by Donnie, which is unlikely but he's loosing his cool a bit because he's technically bullying Casey.

"Is this Jersey..?"
That was the elder turtle and he sounded.... Just. like. Leo.

Donnie freaked out about this throws the taser at this old turtle.

"Where- where am I?!"
He says as he looks at his tied self.

"Sensi?"
Casey says, and the turtle's eyes lights up.

"Casey...wait! Is the Kraang still here!?"
He says quickly.

"No it's been three months since the invasion."
Casey responds, okay this really sounds like Leo.

"Three months..? Three months!? Did- did you beat the Kraang!?"
He says lighting up even more.

"Yeah and- and you-..."
Casey trails off, everyone's expression saddened, while Donnie's soured.

"How do you know these things you Doppelgänger!"
Donnie points his bo-staff at the older looking Leo.

"Woah woah woah! We are not hurting him, I don't want to lose him again!"
Casey says clenching his fists.

"Wait did I...did I die?"
The older looking Leo's eyes of joy replaces with shock.

"Y-"

"Yes now stop impersonating him before I smite you!"
Donnie says creating a small mystic drill with his bo-staff while interrupting Casey.

"Wh-this is a misunderstanding Donnie, I'm from the future- well a different fabrication of s timeline."
The older Leo says.

"Th-that's not possible! You're- you're supposed to be dead!"
Donnie grumbles, but he's starting believe.

"I thought so too- I- I don't even know why I'm here-"
He says.

"Introduce yourself like Leo would be if he was Leo."
Donnie scowls.

"Oh great okay.."
He clears his through.

"Yo what's up my broskis! Having fun in the Kraang invasion? Because I certainly don't like it!"
The elder Leo does a very good impression on his younger self.

"That was..."

"Surprisingly accurate."
Mikey says wiping away tears, he's already missing Leo, he grabs Donnie's little pocket knife and cuts the ropes.

"Hey-!"
Donnie yelps.

"You really are Leo! Did you time travel through the prison dimension!? Or what happened!?"
Mikey says hugging the older Leo.

"I- I don't know I was floating in a middle of nowhere."
The older Leo replies to Mikey.

"It doesn't matter."
Mikey squeezes the older Leo tighter, the older Leo seems to be holding back tears.

"Oh Mikey..."
Raph says he's practically in tears as he also hugs the older Leo, Casey too.

"N-no! It's impossible! This- he's just messing with your head!"
Donnie scowls angrily.

"Baby blue?"
It was Splinter, he sees Leo his eyes filling up with tears.

"Master Splinter..."
The older Leo says in a formal matter, he can't help but cry now.

"I'm sorry- I- I didn't do better.."
He sobs hugging the four of them.

"Oh...."
Splinter hugs the older Leo.

Donnie sourly stands away from the group hug, Splinter notices it.

"Purple? Why aren't you welcoming your brother?"
Splinter asks with a raised eyebrow.

"That... that Doppelgänger is not my brother."
Donnie says bitterly.

"Donnie."
Splinter says more sternly.

"Sure he's bigger and wrinkly but you don't just revive from the dead-"

"Donatello."
Splinter's voice is even more stern.

"You can't magically life again! Plus Leo's at that graveyard and-"

"Donatello Hamato you come here this instant."
Splinter says his full name which means he means business.

"Ugh."
Donnie says and joins the group hug, as much as he didn't want to admit it he really missed it, it was just like Leo's hugs. Donnie couldn't help but burst into tears.

"Woah, okay getting emotional Don."
The older Leo chuckled patting his back gently.

"I-I'm not emotional! It's just all to you are onions."
He sobs.

After a long while Donnie decided to visit Leo's grave with the older Leo.

"So. This is where you buried me?"
The older Leo says putting his hands in his long coat's pockets.

"Yeah."
Donnie says as he places some white roses on top of his brother's gravestone.

"My last words better not be cheesy if I died from the Kraang."
Donnie heard the older Leo mutter under his breath.

"No. He said it wasn't our fault. But it was someone's fault."
Donnie says clenching his fists in anger.

The older Leo cleans the snow off the gravestone.
"Huh better than nothing."
He says then looks at Donnie with worry.

"If I- him- you know- said it's not your fault then it's not."
The older Leo reassures Donnie which only made him a bit more upset.

"It-it was!"
Donnie protested.

"Who? The Kraang right?"
The older Leo crosses his arms.

"No. Just. Me. It's my fault."
Donnie says his voice breaking.

"You're fault? How?"
The older Leo's eyebrows quirked up.

"I- I didn't help you- Mikey opened a portal to save you- Raph grabbed you- but...I just choked and froze."
Donnie says painfully, sympathy shines in the older Leo's eyes.

"It's really not your fault."
He says.

"It-it is! I-I'm so useless- what kind of brother am I if I can't even save Leo?! What kind of- hero!?"
He starts to cry a bit just looking at the older Leo and his gravestone.
The older Leo sighed, and gives Donnie a hug.

"You...you died in my timeline. We fought a lot. But...I regret what I said to you, I said that I hated you. I regret those words."
He says his eyes were watery a bit.

"What-why!?"
Donnie says in slight shock.

"I said I'd hate you if you died in my hands. And you did."
He sobs slightly.

"That's the same with me...I was yelling at Leo- Err you or whatever! I just told him if he died I'd hate him. He didn't exactly die in my hands but he did die.."
Donnie wipes the tears away from his eyes.

"I buried you in this exact same place, with the exact same place, next to...someone else."
Leo says sadly.

"You mean..."
Leo nods.

"Oh my god...I'm so sorry."
Donnie says squeezing the older Leo tighter.

"No I should be saying sorry to you."
The older Leo cries.

"I should've known."
They say in unison as they hug next to the exact same parallel gravesite.

"I'm sorry for saying that."
They both say again, as the graveyard seems more happy even Donnie.

Okay anyways end of story

Can totally make more but uhhh I suck at making angst Hah.