Chizome was an intimidating, muscular man who walked with a distinct hunch and had a flat, somewhat triangular face without a nose, which he removed himself, and an extraordinarily long tongue tainted with numerous small papillae. He had long black hair, which he wears messily; it would drape over his face if not for his headband and mask. He had small red eyes (white in the manga) with tiny irises.

Chizome sported a dark combat suit, plated with metal armor across his body to holster his weapons. The torso was sleeveless, so he wrapped his arms in yellow bandages from just below his shoulders to his wrists. Chizome also wore long black wristbands and a watch on his left arm, and his knees were protected with metal pads; black boots with steel armor and spikes completing the costume. He always donned his signature blood-red scarf, matching headband, and the tattered cloth he wore around his face as a mask.

After being freed from Tartarus by All For One and returning to society alone, Stain had donned a white jacket with oversized sleeves. His forearms were wrapped with bandages, and he wore red gloves, dark navy blue pants, red metal pads around his knees, and black boots with white pads. He had two belts, one black worn diagonally on his torso where he held an old katana, and the other red worn around his waist holding what appears to be a small machete. His face was covered with a tattered cloth, leaving a part of his hair visible, which was tied into a ponytail.

As Stendhal, his hair was even longer and less messy than it is currently and was tied into a neat ponytail. He wore black and red clothing that looked similar to bulletproof clothing and had six small folding knives, three on the right side and three on the left side, with two pairs of pockets beneath them, but his most striking feature was his mask, which completely covered his face.

Stain was a cruel, unforgiving, and merciless man with a sense of duty to cleanse society of false heroes. He believed that those who become Heroes for fame or money are unworthy of being called the name and only All Might is a true hero. Due to his strong ideology, he took it upon himself to become the Hero Killer: Stain to stain his own hands with blood to purge fake heroes and change the current society.

Stain did not wish to kill those not targets for his cause, and he disliked needless bloodshed, thus explaining why he came to blows with Tomura Shigaraki. He was reluctant to attack Tenya Ida before the hero-in-training provoked him. Stain even told Tenya the essential traits of a hero in contrast to those who desire to use their Quirks in selfish ways, such as Tenya seeking him out to avenge his brother. Stain spared Izuku Midoriya because he shows the characteristics of a true hero: strength and capability, as well as an innate sense of doing what's right. Stain believed heavily in creeds and convictions, stating that they are needed to accomplish anything and that people are weak and will eventually die without them. The Hero Killer even spared Tomura upon noting the seed of a warped conviction within him and their similar goals of destroying the status quo despite the radical difference in their final goals.

Despite his extremist views and beliefs, Stain did have genuine ideals about heroism, which he stood by firmly. He did not hesitate to save people he deemed worthy of being called Heroes, did not like killing without reason, and would even give advice to his enemies. Stain acknowledged that what he did was morally wrong but believed that for society to be put on the right path, someone must be what he was, suggesting that he saw himself as a martyr. This self-righteousness is further supported when he tells the now-retired All Might to come and end his life while saying beforehand that he would only allow All Might to kill him, indicating that he wanted to die at the hands of a true hero.

Stain's conviction gave him an incredibly murderous aura and bloodlust that paralyzed several heroes in fear. It gave him enough confidence to challenge several Pro Heroes simultaneously, even after sustaining significant injuries.

Despite being a Villain and serial murderer, Stain was willing to aid the Heroes in combating more significant threats. This collaborative aspect is shown during the Tartarus prison break, where he didn't join All For One's forces and instead collected information on the escapees while vowing to a dying guard that the data he took would end up in the hands of the right people. While he refused to acknowledge All Might in his skinny and emaciated form, Stain still recognized him as the hero everyone admired, revealing that he never admired All Might for his power but because he kept smiling and did everything he could to help people. Stain also said that he knows of All Might's great legacy and claims that those who carry on his will shall continue to fight for justice.

Much later, Stain went a step further by aiding All Might in his final battle against All For One, showing his willingness to aid the hero he genuinely looked up to. Even after being heavily injured and moments away from death at the hands of All For One, Stain confessed that it was All Might who taught him what being a hero truly meant, going as far as to refer to All Might as his everything. In his final moments, Stain merely smiled and held out his arms in support of All Might, encouraging the hero to live on and win.

As Stendhal, Chizome believed that any who "wielded power without conviction" were automatically sinners deserving of death and used this belief to justify his actions as heroic instead of hypocritical. That said, he was perfectly willing to work alongside villains like Kuin Hachisuka as a means to an end, justifying that there was value in villains seeking to exterminate other villains. Even so, he also had a sense of common ground and respect for other vigilantes, such as saving Koichi Haimawari from an Instant Villain and identifying with Knuckleduster, believing that the latter and he were on the same path of fighting for justice. Despite this, however, he was willing to fight and kill them if they opposed him.

Stendhal also appeared to suffer from a split-personality disorder that caused him to view his vigilante and civilian identities as two separate entities (his mental change was signified by whether or not he was wearing his mask at the time), with him considering Stendhal as something more significant than him. However, his confrontation with Knuckleduster caused him to change his beliefs and start thinking that those who are not "true heroes" are the reason for society's problems.