Keepin' It Ninja

Mikey's easy. Mikey doesn't want any of the hassle that comes with being A Responsible Person. He's a classic enneagram 7 in a lot of ways, unwilling to experience real pain, fear, or boredom if he can avoid it - and he puts effort into avoiding it! Mikey doesn't get away with things because the rest of the family can't help but remember him as an adorable baby and want to spoil him; he gets away with things because he's a conniving little shit (affectionate) who has guaranteed there are consequences for asking Michelangelo to do anything he doesn't really wanna. "Would you rather have Mikey happy, or Mikey bored?" Bored Mikey is making it everyone's problem. He whines, he sulks, he goofs off, he deliberately does jobs badly so people won't ask him to help... and every now and then he has to eat his own crow when the rest of the family dismisses or knuckles down on him as a result.

It's not a mindset that can really develop without some amount of enabling, of course, and Mikey wouldn't be Mikey if he didn't have three brothers and a father exasperatedly willing to take up at least some of his slack. He accepts their lead because leadership is earned, and he'll cheerfully shrug and admit he's never tried to earn it; they have, and he trusts in that with so little question it borders on hero worship at times. The pushbacks he gives are the pushbacks of his entire life philosophy - does it really have to be this serious all the time, dudes? is carrying around all that worry and anger really helping any of us? - but he's self-aware enough to be able to articulate the reliance he has on his family to shoulder stresses for him. It softens his selfishness: he respects his brothers, the disciplined Leo most of all, and his faith in them is a powerful thing. When things get dire, they're the ones who'll know how to fix it, and he'll do the part he's assigned.

Leo is similarly fixed in his position through both nature and conscious choice. "Tales of Leo" shows us a kid who was already the hall monitor of the family, the reliable one, the worrier, more inclined to reinforce the rules than break them and forever keeping an eye on the others. He takes himself pretty seriously, for both good (a hard worker who has genuinely mastered his art and has depths of emotional resilience to draw on) and ill (a condescending bossypants who experiences failure as a blow to his core self-identity), and it seems to have been that way well before the show began.

We never see the moment Leo is officiated as The Leader, so we don't know for sure how much nurture was involved leading up to to it: whether there was a point in their childhood where Splinter often used to leave Leo in charge when he was away on trips, whether he always planned for one of the brothers to step in as head of the family in preparation for the inevitability of his own passing, or whether it was strictly meant to be a hierarchy for above-ground excursions and the risk of conflict that slowly became more of a norm than Splinter ever expected. Whatever the case, by the time we meet him Leo himself has clearly accepted that he is the leader and spends the rest of the show actively pursuing that role. Because they're a team and a family, the more Leo grows into this position - and he does grow into it! check out how much pushback he receives from his sibs in the very first episode versus after "Tales of Leo" - the more the others accept it as part of their domestic hierarchy as well, though there's always room for some ribbing if Leonardo starts leaning on them a little too heavily. They're unsettled and upset whenever the role is vacated. They rely on him to be who and what he is, and support him from their own places.

Don and Raph, meanwhile... in my early fandom days it was very widely accepted that Raph was the 'second eldest', and that's not without reason. Unlike Mikey, Raph does want to be in charge for at least a little while, it's suggested he's stronger and possibly bigger / bulkier / hardier than the others, and even after he concedes the role of leader he maintains his niche as protector. The responsibility Raph takes on himself is very simple: this is his family and no one messes with it (except him). Doesn't matter if the threat is physical or emotional, external or internal - if something is seriously fucking with his family then he ain't having it. He'll slap down killer robots and sibling self-recrimination with the same unflinching determination. It makes him a sturdy shelter the others can turn to in times of distress, and a quick responder in a crisis.

What holds him back is that he frequently shirks the responsibility to take charge of himself. Raph rides the highs and lows of his emotions without brakes, and it makes himself reckless and impulsive in ways that could be genuinely dangerous - if he didn't have three brothers there who'll usually check him before it gets to that point. "Raph's free not to think," Mikey points out, and it has to be acknowledged he takes liberties with that freedom, the same way Mikey takes liberties with his license to slack off. Sometimes he's the one who's messing with his family, and sometimes he's aware of it, and sometimes he doesn't do as much about it as he could. Raph undermines his own value as a 2IC every time Leo has to devote his focus to yanking him back into line. It's definitely something that improves as they all grow up, but it's also just an accepted truth in the family hierarchy: someone has to be ready to keep Raph from going off half-cocked.

On the flipside, Don has taken on a huge amount of self-appointed responsibility as the family technician - he maintains their home, builds their equipment, and provides invaluable advice in the field. I've referred to Donnie as the strategic 2IC a few times because, genuinely, he knuckles down with Leo a lot to hash out their plans, be they the rare times they get to actually lay out a strategy or on-the-fly reactions to the strange things they encounter day by day.

At the same time Don, like Mikey, doesn't really seem to want the specific responsibilities of leadership. His inventions are a passion, not just a necessity, and he likes to have the time and space to devote to them, and to his learning, and to his friends. He doesn't want to be constantly keeping an eye on everyone, or always having to be the voice of fretful reason over the voice of 'that seems kind of fun, though'. He outright abdicates from intervening in family conflicts sometimes, a la his iconic "ooh, I'm not touching that one" moment. Leo and Raph are fighting? Don and Mikey are rolling their eyes at each other about it. No thanks. Not his bag.

I wouldn't call Raph and Don twins myself, but I also don't know that I'd portray them as having the same kind of consistent hierarchy with each other as they do with Leo and Mikey. It's complicated! Raph's role as protector makes him more of a watchdog by simple nature, consistently covering Don's back on the battlefield, worrying about his emotional state, being the one turned to in a pinch (the equally iconic, "Raphie, we've got trouble down here..."). Don will take charge, though! He and Splinter call a lot of the initial shots in "Shredder Strikes Back Part 2"; he drives most of what they do in "Return to the Underground"; he is calling the hell out of those shots during "H.A.T.E". And Raph doesn't object when he does. Don's smart; if he says this is how it has to play then, well, most of the time Raph goes with it unquestioningly. There's some real admiration there.

The fall of the Lair in "Prodigal Son" is another interesting insight into their dynamic. Don effectively decides that they need to retreat, but he won't do it without Raph's agreement. It takes both of them to make a call on that scale, and though Raph clearly takes on the ultimate burden of being the one to pull the trigger - and thus carries the most anxiety about the consequences - he needs that check from Don to be able to do it. He needs that reining in from his own aggressive impulse.