Attack on Titan's combat system is truly unique, pitting soldiers with paring blades and ODM gear against titanic humanoids who are only vulnerable on the napes of their necks. Later revelations made it clear that some people can turn into Titans at will, and nine unique Titans exist in the world. But they don't always look the same as their wielders.
Eren Yeager was the first shown Titan Shifter, becoming the Attack Titan in the first season to save the day against the Titan onslaught. More Shifters soon appeared, from Annie Leonhart's Female Titan to Zeke Yeager's Beast Titan, but why aren't these unique Titans more visibly consistent in relation to their users? Some theories may help explain this.
When a Titan Shifter Abandons Their Human Appearance
Some of the Titan Shifters have similar appearances between their human and Titan form, while others do not. Annie Leonhart, one of the Marley Empire warriors, certainly looks much like herself once her Titan powers are activated, from her distinct feminine body to her facial expressions and blonde hair. Similarly with Reiner Braun, with his short blond hair and his masculine body. Eren is a bit looser with this rule, having a male form but pointed, elf-like ears and shaggy brown hair.
Most pure Titans look very similar to the original people they once were, even including the distinct Titan with a gray beard from early in the show (and his similar human form was shown in a flashback). But there are also Titan Shifters who look markedly different in Titan form as compared to human form, with the Marley warrior Pieck being an example. She is a petite young woman with long, dark hair, but her Cart Titan form is male, with short hair and an elongated horse-like head. Lara Tybur's War Hammer Titan had a male form and no hair at all, meaning Lara's true appearance was a total surprise when she was extracted from her Titan during Eren's raid on Liberio. How can Annie look like herself in Titan form, but not Lara or Pieck?
The Many Factors In A Titan's Appearance
Some Attack on Titan fan theories may help explain why some Titan Shifters resemble their human forms while others do not. The Jaw Titan is a good starting point, with Porco, Falco, Marcel Galliard and Ymir Fritz being four known wielders of this form. As per its name, the Jaw Titan will always emphasize its jaws as its weapon, but Ymir's Jaw Titan did not resemble the version seen in Season 4. Ymir Fritz wandered Paradis Island as a pure Titan for an impressive 60 years, finally happening upon and eating Marcel. She obtained the Jaw Titan's power, but it closely resembled her pure Titan form, rather than the distinct Jaw Titan form of Falco and Porco later on. Perhaps Ymir grew so used to her pure Titan form that assuming a new one would have been an overwhelming shock, so her body instinctively retained its form despite gaining new powers. The Jaw Titan's other wielders had no such issues.
Then there's the matter of Pieck's Cart Titan, which moves on all fours while carrying large items on its back. Pieck's human form is utterly absent in this Titan, suggesting that either she pragmatically wanted a male form to avoid having her breasts drag on the ground, or the Cart Titan is permanently male, and Pieck had no choice but to have that form. Similarly, when Armin Arlert was granted the Colossal Titan, his form closely resembled Bertolt's, rather than his short-lived pure Titan form. Most likely, that's what the Colossal Titan is "supposed" to look like, with function taking precedent over form. Another example is the War Hammer Titan, having a hairless male form rather than the distinct female form of Lara Tybur.
But other Titan Shifters do resemble their human forms, such as Annie's Female Titan and Reiner's Armored Titan. Perhaps these Titans have more leeway with personal customization because their role is so simple that their appearance doesn't interfere with their function. So long as the Armored Titan has armored skin, its gender and hairstyle don't matter, while the Female Titan is an agile and nimble fighter, no matter its hairstyle. With these Titans, the Titan Shifter form may be based closely on the user's original pure Titan form after getting the Titan serum, which in turn, typically resembles their original human form (but not always). If so, perhaps Pieck and Lara ended up with male bodies as pure Titans, and thus their Titan Shifter forms were also male. This is another compelling theory concerning their Titans, since pure Titans nearly always look male. There is no precedent for a man gaining a female pure or Shifter Titan form as of this point.
There may also be a minor theme of genetics in Attack on Titan's Shifter forms. Eren, Grisha and Zeke have all become Titan Shifters, and their common trait is their sharp, elf-like ears. This may suggest that the Yeagers have strong genes, or the author simply did this because the Yeagers are an important family to the lore, and many of them are protagonists. Having this consistent trait helps reinforce their hero status to the viewer.