Ulquiorra Cifer

Ulquiorra is a slender, yet fairly muscular, male Arrancar of average height with a melancholic appearance. He has fairly short, messy black hair, pale white skin, a black upper lip, and green eyes with small slit-shaped pupils, similar to a cats. Small green slits going down his cheeks below his eyes as they stop half ways.

He was an Arrancar, a hollow that has removed its mask and gained humanoid form, and served as the 4th Espada in Sosuke Aizen's army. Ulquiorra possesses tremendous physical strength, speed, and agility, as well as powerful energy-based attacks. Ulquiorra is an extremely cold and callous individual with a very brooding and indifferent attitude, willing to kill both enemies and comrades that get in his way. He refers to anyone he finds uninteresting as trash, fully expendable but is not extremely violent unlike other Espada.

Ulquiorra, the fourth-ranked Espada, represented nihilism. Unsurprisingly, Ulquiorra was an empty shell. Not only was he unable to feel or show emotion, but he also didn't understand them. As such, he carried a fatalistic worldview and cared little for those around him. but can You change him? can you make him feel more human?

Ulquiorra as a Hollow. As a Hollow, Ulquiorra felt alone in the world, being white while those around him were black. He wandered Hueco Mundo alone, lamenting his existence, until one day he came across a white tree with many sharp spines.

In the Bleach universe, a hollow is a monstrous soul-eating spirit that results when a deceased soul is consumed by negative emotions or is forcibly converted by other hollows. (another explanation)
Hollow are former Pluses (deceased Human souls) who lose their hearts to despair or regret, or they remain in the Human World for too long. Any spirit who is not guided to Soul Society by a Shinigami via Konsō may eventually turn into a Hollow.

Arrancar are a group of Hollows that gained similar powers to that of a Shinigami by removing their masks. The individuals that Aizen turned into Arrancar using the Hōgyoku generally possess powers far beyond those of previous Arrancar, with some exceptions like Coyote Starrk and Ulquiorra Cifer.

I think he looks about the same age as Ichigo and co. Considering he has a more curved face which indicates youth most of the time in anime, large eyes, and he's short.

Actual Age: 300 (if we’re counting from when he was an arrancar)
Apparent Age: 19 (right at the tail end of his emo phase)

Ulquiorra's most powerful ability is his second release, "Segunda Etapa," which transforms him into a more powerful form, giving him an even greater boost in strength and speed, and the ability to create powerful energy blasts capable of destroying entire
In his released state, Ulquiorra becomes bat-like in appearance. While in this form, large, black bat wings form on his back, his hair grows longer and wilder, and the remains of his Hollow mask centers atop his head, with two large horns extending outward to the sides towards the front.

Enhanced Strength: Ulquiorra is deceptively mighty for his slim build. He can physically injure Yammy, an Espada known primarily for his physical strength, with no visible effort. He can upturn huge portions of rock floor with a single strike at the ground with his sword.
He is a master swordsman with immense spiritual power, and he was the only Espada capable of using a second Resurreccion. Ulquiorra was insanely powerful – to the point that there are only a handful of characters who are strong enough to beat him

Ulquiorra cifer has a hole that goes right thru his chest. it's a bit small though, The hole is a physical manifestation of the loss of the hollow's 'heart', which in turn becomes the mask the creature wears.