Considering that Bleach ran for over fifteen years, it's not surprising that it has a whopping 686 chapters and 366 episodes to date — the manga's Thousand Year Blood War arc is currently being adapted after a five-year break. There are hundreds of named characters, many of whom appear in a single chapter.
Likewise, there are quite a few characters that have had significant roles in the Bleach narrative: the series' antagonists being the major game-changers. The Shinigami of Soul Society fight off one crisis after another, a deluge of war preceded by centuries of peace. The story villains include Hollows, Arrancar, Espada, Quincy, and even treacherous members of the Gotei 13.
Coyote Starrk
As one of the strongest Espada, Coyote Starrk is overwhelming long before he fuses with Lilynette Gingerbuck, which consequently triggering his Resurrección, Los Lobos.
In this form, most Shinigami captains are little more than fodder for him, as Rose and Love find out when they attack him. It's truly a matter of luck that Starrk's main opponent turns out to be Shunsui Kyoraku, whose shikai, Katen Kyokotsu, is too versatile and too complicated for the Espada to handle.
Askin Nakk Le Vaar
Askin is taken to the Royal Realm along with Yhwach, proving the worth of his Schrift — and, more importantly, the skill with which he wields the Deathdealing. His combat prowess, considerable enough to give the likes of Urahara and Yoruichi pause, is vastly augmented by his Vollständig, Hasshein.
The sheer number of "Gifts" that Askin throws at Urahara forces the latter to activate his bankai, Kannonbiraki Benihime Atarame, for the first time in the story. The Shinigami technically wins, but his ultimate fate is left ambiguous in the manga.
Yammy Riyalgo
Originally listed as the 10th Espada, Yammy reveals that his true power is awakened only when the rage "stored" in his body begins to burst at the seams, displaying its devastating strength via his Resurrección.
One of the bigger problems with Yammy's Ira is that his opponents become nothing more than insects to be swatted at (when he mutates into a truly gargantuan monster with several extra limbs). Although clearly a difficult opponent, Yammy is effortlessly taken down by the sublime combination of Captains Byakuya and Kenpachi.
Gremmy Thoumeaux
Gremmy would have been a lot more powerful if his body had the capacity to match the breadth of his imagination. Suffice it to say, his Visionary allows him to bring literally anything he thinks of into existence, whether it's recuperating from damage or generating offensive attacks like volcanic lava, machine guns, and a meteor half the size of Seireitei.
Gremmy is best known (and feared) for imagining more versions of himself, who then proceed to amplify his Visionary accordingly. Too bad he has to face Kenpachi in the end.
Pernida Parnkgjas
Pernida Parnkgjas is the Left Hand of the Soul King, and naturally possesses techniques that Yhwach generally offers to his minions in the form of Schrifts. Its Compulsory works as a nervous puppet-master of sorts, using "nerves" to govern its enemies' anatomies with near-perfect precision.
Pernida's raw power results in Kenpachi losing his arm, although this is because the Squad 11 captain tears it off himself. In addition to its durability and tenacity, Pernida can move faster than Yoruichi can react, which is most impressive.
Lille Barro
The so-called first of the Quincy, Lille Barro, proves a formidable foe when he takes on Shunsui Kyoraku, who barely manages to eke out a victory with the full might of his bankai. Unfortunately, not even Katen Kyokotsu: Karamatsu Shinju is a match against Lille's Vollständig, Jilliel, an angelic entity that is immortal and invulnerable in every conceivable way.
The Quincy calls himself a god, an arrogant declaration that results in his doom when Nanao Ise reflects his "divine" powers with her spectacularly unique "zanpakuto", Shinken Hakkyoken.
Gerard Valkyrie
Gerard Valkyrie's fearsome ability, The Miracle, somehow permits him access to what he refers to as "miracles", which is vaguely similar to exploiting probability itself. In fact, attacking him directly works against his opponents because he merely transforms their offensive force to augment his various battle stats.
Gerard's sword, Hoffnung, is practically indestructible, not to mention effortlessly deflecting any attack directed at him. And then there's his Vollständig, Aschetonig, so strong that it is only destroyed when Yhwach absorbs Gerard.
Jugram Haschwalth
Yhwach calls Jugram Haschwalth his "other half", an epithet that isn't taken lightly by either of them. Not only can Haschwalth magnify his comrades' powers, but he can also use his master's Almighty when the latter "sleeps" for the night.
His Schrift, The Balance, works on the principle of luck transference — Haschwalth is able to manipulate the abstract concepts of fortune and misfortune in such a way that he is the sole beneficiary of the process. He never displays his Vollständig, but it's not like he needs one anyway.
Aizen Sosuke
Aizen plays the role of the overarching villain for the longest time, starting with his defection from Soul Society and ending with his defeat by Ichigo's Mugetsu (and consequent imprisonment.)
His Hogyoku skillset notwithstanding, the reason that very few people are capable of battling Aizen without being instantly killed is a combination of staggering reiryoku and Kanzen Saimin. His zanpakuto, Kyoka Suigetsu's Perfect Hypnosis cannot be avoided, not even by Captain-Commander Yamamoto Genryusai.
Yhwach The Almighty
The concluding antagonist of Bleach turns out to be the Quincy originator, Yhwach, who is resurrected 999 years after his previous "defeat" at Yamamoto's hands.
The Almighty allows the villain to observe a near-infinite multitude of future timelines as well as winnow the options down to a satisfactory window of opportunity. In other words, Yhwach has absolute control of nearly every situation. If this isn't terrifying enough, he gains an unknown power upgrade after ingesting the Soul King (his father).