Typically, it is rare for doctors to fulfill roles other than medical practices for the benefit of those around them, but as the Straw Hat Pirates' doctor, Chopper's responsibilities are not limited to just medicine alone.
He occasionally performed tasks like steering the ship as their unofficial helmsman while on the Going Merry, and during the heat of battle, he can also fulfill the role of a physical fighter to support his friends.
However, due in part to his lack of battle experience or formal training (as well as his Rumble Ball's time limited effect) Chopper was originally considered part of the "Weakling Trio" along with Nami and Usopp.This lay largely in his lack of battle intuition and naïveté at the time, though it is shown that he has gotten much more capable after his two years of training on Torino Kingdom.
In spite of his usual fearful behavior, Chopper is shown to have a strong will as he withstood a burst of Haoshoku Haki from Big Mom.
During the Whole Cake Island Arc, Chopper showed great tactical skills in the middle of his confrontation with Charlotte Brûlée as he and Carrot came up with a method to deceive her by using a decoy of Carrot created by Brûlée's power. He has shown to be very analytical, as he examined his surroundings before taking action. He pretended to remain restrained by chains in his Heavy Point, and when Carrot toppled the soup towards Brûlée and electrocuted most of the enemies, Chopper used the element of surprise by changing into Brain Point to break free before using Monster Point, subjugating the Crocodile homie and wiped out the remaining enemies. Chopper even thought of exploiting Brûlée's Devil Fruit ability to transverse Big Mom's territory through the Mirro-World in order to find and save his friends while remaining undetected by a majority of the Big Mom Pirates. His strategy received praise from Capone Bege, the captain of the Fire Tank Pirates and a master tactician.
During the mission to rescue Brook, Chopper used a skeleton with sea weed for hair to use as a replacement for Brook after they retrieved him from Charlotte Linlin.
Because he is originally an animal, Chopper can understand and communicate with other animals, thus allowing him to serve as a translator between animals and humans. This was first seen when Chopper translated Karoo's speech on how he ended frozen in the icy waters of Drum Island next to the Going Merry.
Having studied under Dr. Kureha, one of the finest doctors of the medically advanced Drum Kingdom, Chopper himself is an extraordinary doctor with outstanding knowledge of both medicine and medical drugs as well as the preparations of most remedies and ointments that can be made from regular fruits, herbs, roots, and vegetables that can be found and used on most islands. He is also capable of administering surgery and resuscitation. Overall, Chopper can treat even the most brutal injuries.
During the two years he spent self-studying the Torino Tribe's massive collection of books on herbalism in the Torino Kingdom, Chopper's knowledge of pharmacology have immensely increased. In the Torino Kingdom, Chopper self studied many books of new and highly effective remedies, ointments, unknown herbs with great healing properties and other medicinal drugs that were developed by the inhabitants of the Torino Kingdom which are largely unknown to the rest of the world. With this new understanding of medicinal drugs, combined with the medical skills he had already mastered in the Sakura Kingdom, the scope of Chopper's healing skills as a doctor have become noticeably wider. The greatest example of this is during the Zou Arc, as Chopper, with the assistance of the Mink Tribe's doctors, was able to use all of these skills to treat the gigantic, thousand year old elephant Zunesha, whose leg was badly injured by the Beasts Pirates.
Chopper was able to invent his very own drug known as the "Rumble Ball", through 5 years of research, which helps to increase his Devil Fruit's fighting capabilities. In addition, while he was under the apprenticeship of Dr. Kureha, Chopper became aware of the highly addictive drug NHC10 (which only selected scientists in selected countries are allowed to use), and its harmful side effects. He is capable of stabilizing the condition of someone suffering from an overdose of the drug. Chopper also easily created a cure for one of Queen's deadliest viruses, the Mummy virus, doing so in a relatively short amount of time. Furthermore, Chopper knows how to engineer viruses of his own, but only with the intent of counteracting other viral diseases.
He has been shown being able to give successful blood transfusions (even between two different species: a human and a fish-man, though human and fish-men do carry the same blood), as he did to save the lives of both Sanji and Luffy, after too much blood loss (even knowing their correct blood type: Luffy's blood type being F (the same as Jinbe's), and Sanji's blood type being S RH- (an unusually rare blood type)).
Masui (麻酔,Masui?, literally meaning "Anaesthesia"): Chopper injects a substance, possibly an anesthetic, into the opponent's body making them faint. This was first used on Luffy when he had a hallucination of a tsunami about to hit them while walking to Yuba in the Arabasta Desert, and it was later used in order to stop a group of rampaging giant children on Punk Hazard, with some assistance from the G-5Marines. In the English manga this technique is called Sedative.
Chopperphage Nebulizer (チョパファージ霧砲,Chopafāji Neburaizā?, literally meaning "Chopperphage Mist Cannon"): After developing Chopperphage, a countervirus that nullifies the effects of Queen's Ice Oni virus, Chopper fires it into a crowd with a cannon, releasing a mushroom cloud with his face on it. This allows Chopper's virus to be inhaled by those in the vicinity, curing them from the effects of Ice Oni. The term phage is derived from bacteriophage, a type of virus used to treat bacterial infections via phage therapy. A nebulizer is a device used to administer medication in the form of mist.
Video Game Techniques
Art of Compounding (,Art of Compounding?): Chopper pulls out a bowl to grind up ingredients to make medicine that can heal his allies. Used by Chopper as his Unique Special Action in One Piece: Burning Blood.
Chopper ate the Hito Hito no Mi, a Zoan-type Devil Fruit that allows him to transform into a human and reindeer-human hybrid at will. It also gave him human level intelligence and the ability to speak human language. It also allows him to walk upright on two legs and hold things in his cloven hoof hands. This power is what allowed Chopper to become a doctor and learn about medicine in the first place, which is impossible for a normal reindeer.
While Chopper is usually seen in his hybrid form, which is a tanuki-like chibi-creature, he would sometimes transform into his human form to try and fit in with humans, often with minimal success due to his full human form looking more like an apeman.
In terms of combat, his full human form is very bulky and strong, and with his humanoid body structure, he can use melee attacks that he cannot use in his true reindeer form, such as punching and kicking.
In addition to his three forms granted by his Devil Fruit, Chopper has also invented a yellow jaw-breaker like drug called the "Rumble Ball", through his research. This drug has the power to "distort the wavelengths" of the Devil Fruit's transformation, allowing him access to additional transformations. The effect of the drug only lasts for three minutes.
Due to the risk of this alteration, the Rumble Ball's consumption must be closely monitored, and for this reason Chopper cannot consume more than one every six hours. If he takes a second one, he would gain the extra transformations, but he would not be able to control which of his seven forms he takes when he transforms. And if a third one is taken, Chopper transforms into a massive, super strong monstrosity, known as Monster Point, which causes him to lose his mind and go berserk. This form also requires lethal amounts of energy to keep active, making Chopper a danger to both himself as well as others, friend and foe alike.
After the two-year timeskip, Chopper has developed the ability to access the additional forms at any time and without time limit, without even needing to actually take the Rumble Ball. Furthermore, both his Horn Point and Guard Point have developed considerably; both his Horn Point horns and his protective Guard Point fluff are much bigger, thicker, and stronger. He has also developed an entirely new transformation known as Kung Fu Point, he is seen being able to perform formidable martial art maneuvers with this new form, melding the advantageous characteristics of Arm Point and Jumping Point.
Additionally, he also developed his Monster Point; he now only needs a single Rumble Ball to enter this form, and he can stay in control of himself, allowing him to fight in his most powerful form for three minutes. How he managed to advance his level with the drug is currently unknown.
Later with Caesar Clown's help, Chopper was able to lengthen the duration time of the Rumble Ball to thirty minutes, allowing him to extend the usage of Monster Point, despite the currently unknown additional side-effects due to this improvement.
As a reindeer, Chopper was naturally born with high levels of cervid speed and agility. After consuming the Hito Hito no Mi, Chopper gained enhanced human physical potential added to his own. He has extreme levels of superhuman endurance, able to withstand powerful attacks from CP9 operative, Kumadori, and even when badly injured, Chopper could force himself to transform into Monster Point and ultimately survived the immense strain, despite his injuries and fatigue that left him unable to move.
During the two years he spent in Torino Kingdom, Chopper underwent extensively intense physical training to further increase his natural physical strength in his based form, which resulted in vast improvements of his Zoan Devil Fruit enhanced physical abilities whenever he transforms. His Heavy Point is now powerful enough to counter a hammer strike from a energy steroid enhanced Dosun, as well as holding back a door against a group of rampaging gigantified children. His new Kung Fu Point form possesses the extreme punching strength of Arm Point while retaining the immense agility of Jumping Point. His Brain Point, which is his weakest form, is now strong enough to easily carry two large iron balls while running at full speed to escape from a murderous Charlotte Brûlée, while his strongest form, Monster Point, is now powerful enough to violently thrash Queen, one of the strongest members of the Beast Pirates who possesses an Ancient Zone Devil Fruit and a bounty of 1,320,000,000, with only a slap while also scarring his allies, including the very strong ones, such as Marco and X Drake.
In addition to the various transformation of his Devil Fruit, Chopper is also a powerful self taught hand to hand combatant. He is very skilled in boxing and wrestling which he typically combines with the increased physical strength of Heavy Point to defeat numerous enemies with ease. With Kung Fu Point, he can use Kung Fu, which he used to defeat several members of the Ammo Knights, which consist of fish-men who possess ten times the strength of ordinary humans.
Chopper's immense medical knowledge of the bipedal anatomy has also proved useful when the Straw Hats were trying to bring down Oars, during which time, Chopper formulated several theories on where it was best to target the colossal adversary, and finding the giant's weak spot. And while naïve, he can also be quite a capable strategist, best seen in his fights against the Mr. 4 and Miss Merry Christmas pair and Gedatsu.