Ren

2012

Leo has the most noticable scaring out of all his brothers! There is deep gouge marks on his right knee, sort of all over the place and just messy in general and the knee is slightly misshapened (due to it being crushed and then stabbed multiple times). His plastron also has three gouge marks across it from Shredder's gauntlets and there is a tiny scar on his throat from his throat being crushed/cut. Not noticeable unless he lifts his head up to show it. He also has scarring on his right shoulder that looks like shark teeth from being bit there.

Raph's left eye is a few shades lighter than his right because he's blind there! He also has a small spiral scar on the right side of his temple just under his mask from the brain worm cutting into his skin. Another brain worm scar is on his wrists and ankles! He rubbed off a lot of skin while struggling in the restraints and it left a mark. He also has a lot of scars on his knuckles from busting them open during fights and when I imagine him/write him, where that chip in his shell is there is also a wide scar across his shoulder that attaches it to the chip in his carapace, like a follow through. As for post his attack from Shredder, part of his carapace and plastron are missing and the chips also spread out like cracks. There's two holes in total!

Donnie had more so burns than lashes! His hands and forearms are pretty discoloured from chemical and heat burns. Also electrical burns on his head from the fourfold trap episode! But his mask largely covers that. This is only noticeable if you stand close to him so doubtful it would be seen in any art, but he has thin scarring all across his body from April ripping him apart! Sort of spiderwebs around him. It will come up at some point, but right now it just isn't notable in the fic. And then he recently cut his thigh open on the glass at the end of frayed so he would still have stitches from that.

Mikey, before the events of the book, has a spiderweb crack on the back of his shell from the letter falling on him from season one. And then he has a canine bite on his left arm from Rahzar. I just assumed his brothers were so focused on protecting him that he rarely got scarred since in my watch throughs, he got majorly hurt the least? Less visible scarring from shackles compared to Raph, but he still has some too. After... that chapter though??? Oh boy. With the bandages off, Mikey is now missing his right eye completely and the wound is scarred over. His shell is slightly cracked and the back of his thighs have minor scars as well. Bruising all over. Not having a good time rn.

Rise

Just because I'm already doing 2012, here are Rise as well! They have less scars to show they have had less battles and because like 2012 Donnie pointed out - they heal a lot faster so most of the listed marks are quite faint unless they are major/recurrent or you are really looking closely

Leo has small knicks on his shoulders from his odaci and katannas! He was clumsy with pulling them out and they left marks until he got more skilled. He also has scarring on his right wrist from the cuffs in season two. He was pulling and yanking on that thing like crazy. Scarring on his right knee now from Shredder slamming his gauntlet into his knee at the end of frayed and minor slashes from breaking through that window and cutting up his skin.

Raph has three slash marks at the top of his shell from defending Donnie from Shredder and it's slightly indented by Shredder stepping on it. His fists are also heavily scarred and his knuckles are constantly swollen and misshapened.

Donnie has slash marks on his soft shell from Shredder ripping into his battle shell and reaching his soft shell. He also has some burn and chemical scars from his lab and down the center of his plastron is a thin line from when SHELLDON tried to kill him and minor shackle marks on his wrists and ankles.

Mikey has a ton of tiny scars, mainly from skidding across the ground and cutting himself on glass/sharpnel. His shell is significantly scratched up from sliding against the ground as well/being thrown around.

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Anyways, hoped that all helps!! I am a huge fanatic of scars and normalizing them - I recently joined the Leo's with my own knee scars after needing two surgeries to fix it lol. If you do make fanart, PLEASE tag me so I can repost it and cry even louder LAHAKSHJSH ily and drink some water!!