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Marco has incredible control of his emotions because emotional fluctuation can impact devil fruit abilities.

He has a weird relationship with pain and with his body in general. His devil fruit does not mitigate pain, even when it heals.

He got his fruit so young he never stopped to question where he got certain ideas or tendencies (like treating other bodies as realistically fragile while treating his own as expendable, going into medicine instead of navigation because he could heal others but not as much as he could heal himself, being protective of his crew and self sacrificial in every area of his life, taking on an identity as the calm one that never wavers so that others could do as they please)

Is Marco who he is, always surviving, ever reliable, ever passive, ever the loyal healer, because he wanted to be?

Does he fight as the Moby’s first form of defense because he wants to, or because only he can walk off a direct hit from a cannon?

Does he have a high tolerance for pain or does he just dissociate because he’s so used to it?

He’s got more bird stuff going on than he realizes/wants to admit. And those aspects (both physical and emotional/behavioral) make romance and commitment beyond his crew hard (in ways he’s rarely willing to acknowledge even to himself).

He doesn’t know the extent of his devil fruit in terms of lifespan, that, the fact that phoenixes mate for life, and his physical aspects (cloaca) make him wary of romance/sex in general. (He did have a thing with Shanks for a bit but Shanks himself has commitment issues in the other direction so they stayed occasional fwb).

Anatomically he’s not strictly human even in base human form and while he often tries to act human he has habits that he slips into from time to time of he’s not actively paying attention.

His gold belt is a single tail feather (the center one of three in his full bird form). He can technically make it go away with the others but it comes back any time he tries to suppress too many bird habits so he gave up and now it’s a belt. It pulls too much to wrap around his waist entirely so he just tucks it into his sash.

Starting from below his shoulder blades he has a strip of feathers that lead to his tail feathers. Also very small soft feathers along inner thighs.

He keeps his hair like that because he has a small crest of thin feathers that are only hidden if he hides them with his hair. The problem is that they only really grow in that spot, so growing out the rest of his hair would make that spot really noticeable. He literally only has hair on his head. That stubble is all the facial hair he can grow.

His bones are actually more fragile than a normal human’s due to being slightly more hollow. This means he actually regularly fractures bones due to the amount of muscle he’s put on over the years (the muscles themselves can put fractures in his bones) so he’s learned to move very precisely.

He’s more still than most people, less wasted movement, similar to certain birds. It also means he’s more graceful when moving in general.

His eyes. Anyone paying enough attention will notice his eyes are weird as hell. To start with he has a nictitating membrane, so he ‘blinks’ by having a translucent membrane sweep horizontal across his eye away from his nose and then retract, all while keeping his eyes open (often while turning his head). It’s VERY fast so unless he’s constantly observed, no one really sees him blink at all. Except when he’s sleepy thats when he blinks using his outer eyelid, but unlike people it’s his lower eyelid moving and the upper one stays put.

His pupils also ‘pin’ which is when he instinctively constricts his pupils, much quicker than human eye dilation/constriction, often to focus on something in a fight. His irises already glow when he’s using his healing or flames, but they also flash yellow if he’s genuinely pissed.

He tilts his head very quickly and precisely (often to the side when he’s thinking and people find it unnerving).

Perches on things. If he finishes paperwork at night he’ll relieve whoever’s on watch and head up to the birds nest (haha) to just. Watch. For hours. He’s incredibly patient. Hands in pockets while he crouches or looms over what he’s focused on is common.

Will make bird noises but he holds them back typically. Hardest ones to suppress are squawks of surprise.

The first time Ace passed out in front of him and woke up, he wiped his face on Marco’s sash without thinking. Marco had him do extra chores for it but internally he found it really endearing, because it resembles feaking, where certain raptors will rub their beaks on a surface to clean off residue and it signals they’re comfortable.

His feathers are only intangible when he’s fighting or using his healing. He’s fully capable of keeping his feathers solid with his flames just being on the ends or barely visible, it’s actually his typical form as a bird, he has to put effort into keeping himself and his body intangible during a fight by constantly focusing. He’s been seen after a fight occasionally using the powder down on his chest in bird form to clean blood off his beak or talons.

So he absolutely molts. He hates it. Typically takes a weeklong ‘trip’ every year to avoid having to put up with it around the crew because he’s bitchy and also sensory wise molting is really only manageable in full bird form. It’s also the only time he has a temper. Interestingly he won’t actually physically lash out at the crew, but he will get SUPER protective and basically sadistic in a fight. Aside from Pops and some of the division commanders, the crew mostly doesn’t know about how deep the bird stuff goes, even when it comes to why he has his yearly ‘trips’. The first division has some vague idea.

He knows his family loves and respects him but the bird jokes (which have dwindled over the years and are never actually hurtful at this point since he beat the shit out of anyone who tried that decades ago) are still annoying. He doesn’t really mind most of the crew calling him a mother hen and even the early-bird jokes are whatever, he just really wants to keep enemies from catching on too much. He’s had to work past being defensive about the bird things over the years and no one gets to poke fun at him aside from his family.