Typically, it is rare for doctors to fulfill roles other than medical practices for the benefit of those around them, but as the Straw Hats' 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.
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, as shown when he managed to save Zoro's life, who was on the verge of death after he absorbed all of Luffy's pain and fatigue from Kuma's Devil Fruit Powers.
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 also easily created a cure for one of Queen's deadliest viruses, the Mummy virus.
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.
Another testament to his intelligence was that he was also able to invent his very own drug known as the "Rumble Ball", through a little research, which helps to increase his Devil Fruit's fighting capabilities.
He has also 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)).
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 has eaten the Hito Hito no Mi, a Zoan-class Devil Fruit which allows him the ability to transform into a human and human-reindeer hybrid at will. However, the fruit has granted Chopper more than just the ability to transform; it has also given him human intelligence, which allows him to both speak and think like an actual human. This 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 human-reindeer 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. But when in combat, he also uses his Devil Fruit powers in order to fight his enemies with the strength given to him by the fruit and usually transforms into whatever form best suits the situation.
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 some research. This drug has the power to distort the wavelengths of the Devil Fruit's transformation, allowing him access to additional transformations for three minutes.
However, 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, his transformations would become out of control. And if a third one is taken, Chopper transforms into a massive, berserk monstrosity (as witnessed at Enies Lobby and Sabaody Archipelago) with equally monstrous strength that requires lethal amounts of energy to keep active, making Chopper a danger to both himself as well as others.
Prior to the time skip, Chopper could transform into five different forms under the influence of the Rumble Ball: Jumping Point, Guard Point, Arm Point, Brain Point and Horn Point. After the two-year timeskip, he is seen being able to perform formidable martial art maneuvers with his new form called, "Kung Fu Point", a melding of Arm Point and Jumping Point, and is now able to produce a new "Horn Point" with larger "Stag Beetle" like antlers which are useful for digging in the ground at high speeds, along with a new and improved "Guard Point" which is now able to grow big and strong enough to protect the Thousand Sunny from one of the Kraken's tentacle's. In addition, he is able to utilize those forms without consuming a Rumble Ball; instead, eating a Rumble Ball now allows him to enter his most powerful form, Monster Point, and fight in it for three minutes. How he managed this is currently unknown.
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 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 results in vast improvements of his Zoan Devil Fruit enhanced physical abilities whenever he transform. 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 Brûlée.
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, as seen in his fights against the Mr. 4 and Miss Merry Christmas pair and Gedatsu.
Chopper, who was originally an animal, is also capable of communicating with and understanding other animals, which has come in handy during certain points in the story. He also has a sensitive nose that can be used for tracking others (except during rainy weather). But because his nose is so sensitive, strong aromas such as perfumes can irritate his sense of smell.
Due to his innocence and lack of interest or sexual attraction to human females, Chopper is incapable of being seduced by human females and it is implied that this trait also renders him immune to the effects of Boa Hancock's Mero Mero no Mi.
Chopper has at least some knowledge of carpentry, as he was seen repairing the damages on the Thousand Sunny after the escape from Totto Land in Franky's absence.
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 Alabasta 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.