Do take note that a lot of the information regarding the Titan Shifters may reveal major story and plot line SPOILERS, including for those who have not read the manga, so please tread carefully.
One of the main reasons why Attack on Titan (Shingeki no Kyojin) is so popular aside from its unique concept is the mystery that surrounds this fallen world. Where did the titans come from? How did humanity end up the way it is? What mysteries lay outside the walls? What are the secrets being kept from the people? These are just a few of the questions that may come up as you're watching the series or reading the manga. Each revelation adds a new layer of mystery and depth that keeps viewers and fans wanting more. While there have been teasers for season two of the anime series, many can testify that even the manga hasn't unveiled many of the story's secrets. Hajime Isayama has truly put a lot of thought into this unique series.
One of the more interesting and mysterious things about Attack on Titan are the Titan Shifters. If you haven't figured it out yet, the Shifters are people who can turn into titans by will. These Shifters have different features compared to regular titans and most of them retain their human qualities while in titan form. They can think, feel, and react as they would if they were human. While in titan form, the human body rests under the nape of the titan (which is the usual weak spot for titans). The human body remains unconscious and vulnerable while in the titan state. Despite having human qualities, Shifters seem to be more powerful than regular titans. All of them possess powers and capabilities not found in other titans which allows them to be unpredictable, dangerous, and also gives them the upper hand in most fights. Their titan blood also helps them heal quickly and even regenerate. One interesting thing to note is that for most of the time, regular titans seem to treat Shifters as if they were human. They'd actually devour a Titan Shifters given the chance.
Titan Shifters seem to age irregularly and it is yet to be confirmed whether some of them were originally humans or titans. A few Shifters have appeared during the series' run and all of them have had a great impact on the story.
Eren is the main protagonist of the story and the half-titan hope of humanity. He was completely unaware of his powers until they were awakened during his squad's first mission. Unlike the other half-titans, Eren's capabilities seem to be growing and he has yet to reach his full potential. Eren's titan blood is directly related to his late father, though he has a very vague recollection of his childhood.
Eren's titan form has quite a different look compared to his human form. His hair grows longer and his facial features are a far cry to from his normal look. Like most half-titans, Eren has enhanced strength and endurance while he is a titan. Eren also possesses one of the most powerful abilities among half-titans called "The Coordinate". This power allows Eren to influence and control regular titans bending them to his will. He doesn't have full control of this power yet. Aside from this power, Eren also acquires the skill of hardening which allows him to harden his body and create hardened structures.
Annie Leonheart - The Female Titan
Another graduate of the 104th training squad (Eren's squad), Annie is one of the more exceptional fighters in their group. She is more of a lone wolf and doesn't get along well with most squad members. She's been trained to fight ever since she was little and has a knack for hand-to-hand combat.
Annie's titan form is also very similar to her in terms of facial features and physique. Her fighting prowess also increases with the stronger and faster titan form. While a titan, she possesses unbelievable endurance and can continuously chase a target for long periods of time. She also has the capability to harden portions of her body for defense or offense. Another unique thing about Annie's titan is that she can call other titans and draw them towards her in a feeding frenzy.
Other Titan Shifters
SPOILERS: this information is only available in the manga, continue reading at your own risk!
Bertolt Hoover - The Colossal Titan
Bertolt is a quiet and reserved person who seems to always stick by his childhood friend Reiner. He is a graduate of the same squad as Eren and was also formerly a member of the Survey Corps before his true identity as the colossal titan was revealed. A lot of mystery still surrounds the origin and home of Bertolt.
Bertolt's titan form is the largest titan to ever be encountered and is the lone 60 meter class titan. His size allows him to rise above walls and gives him enormous strength. Aside from his size and strength he also has the ability to heat his body and nearby surroundings to extremely high temperatures. However, because of his size he moves slower than other titans.
Reiner Braun - The Armored Titan
Another graduate of the same squad as Eren and former member of the Survey corps, Reiner was often looked up to as the big brother of the squad. He is kind and honest, and has a strong sense of duty towards being a soldier/warrior. You'll rarely see Reiner without Bertolt as they are childhood friends.
Reiner's titan form is very similar to his features while he is human. He has most of the advantages of titans in terms of strength but he also has immense speed. His skin is a golden hardened shell that can break through gates and and even repel cannon fire. Like the colossal titan, the armored titan can also emit steam and heat from its body.
Ymir - The Dancing Titan
The graduates of the 104th training squad continue to be decorated with the addition of Ymir. Ymir is a very close friend of Krista though she is a very secretive person. She's mostly cynical and sarcastic unless it is towards Krista. Ymir seems to know a lot about the past and the secrets of the titans and humanity but she does not reveal this to her squad.
As a titan, Ymir is the smallest one among the Titan Shifters, only about 7 meters tall. Her physique resembles that of a chimpanzee and she possesses great agility paired with sharp claws and fangs. Unlike Annie, Ymir's titan form does not have any feminine features.
Frieda was supposedly the heir of "The Coordinate" which she inherited from her uncle. Before she could learn and master the power, it was stolen by Grisha Jaeger.
Frieda's titan form is also a feminine looking titan with white hair.
Grisha is Eren's father and a man surrounded by a lot of mystery in terms of relations with titans and human history. As a titan, he stole Frieda's Coordinate ability which he then passed on to Eren.
Grisha's titan form is similar to that of Eren though he is bulkier and has a beard.
Not much is known about the Beast Titan. Its features resemble that of an upright ape and it is taller than most titans with a height of about 17 meters. It is capable of speech and is very strong and intelligent. It was even capable of hurling boulders at long distances with precision. Reiner even referred to it as the "End of Humanity" and the strongest titan. What's very dangerous about this titan is that it seems to have the capability to transform humans into mindless regular titans. This has yet to be explored as the Beast Titan only appeared for a short amount of time in the manga.
Attack on Titan seems to have a lot more story to reveal and it seems like we have a long way to go especially in discovering the truth about what's outside the walls and the history of this apocalypse. If this hasn't sparked your interest further, I'd definitely recommend you see the series or read the manga for yourself. Hopefully our curiosities will be satisfied soon with more releases and the upcoming second season.