(The encyclopedia of manga tropes will totally exist someday when I am not lazy.)
This character type appears most often in shounen manga. The Tiny Infuriated Bastion of Heteronormativity is a young boy – markedly childlike in appearance compared to other male cast members – whom the narrative makes a point of proving “more manly” than the male protagonist. When he is stoic he is more stoic, when he is angry his speeches are more impassioned, when he is in love he is more awkward and monosyllabic, and when he loses a fight his urge for vengeance is greater.
He will frequently attempt to prove himself better and more masculine than the main male character, particularly if said character has taken away the attention of an enemy or love interest on whom he was himself fixated. He will often be sensitive about his height, and there may be non-consensual cross-dressing.
In the sort of ensemble manga in which the hero leads a group of mostly-male fighters, the Bastion almost always begins as an antagonist, usually a subordinate to some enemy of the hero’s. He will usually fight the hero and scorn his “weakness” but eventually be brought around to his side by some revelation of the hero’s “true strength.”
In this situation, the Bastion almost always has some strong emotional attachment which the villain to whom he is subordinate uses to manipulate him. (Sometimes, later, the hero and his compatriates use the same tactic.) Usually this attachment is female: a mother, sister, or grandmother; the hero’s love interest; or the Tiny Adorable Bastion of Heteronormativity, his female counterpart. A feminized younger brother is also an acceptable object of attachment for the Bastion.
The typical Bastion is obsessed with “becoming strong” so that he can better “protect” the object of his affection, a desire that the villain feeds and uses against him. He is often socially awkward with the object of his affection, inadvertently doing things that hurt her/him, or panicking and refusing to spend time with her/him on the grounds that a suggested activity is unsuited to his dignity. The only method he knows of expressing his feelings is defending the object of his affection from attack. She/he can usually see right through this. (Unless she/he is the hero’s love interest, who is required to call him “cute” and be oblivious to his feelings.) There will be a scene in which he overhears her/him praising his “kindness” to someone else, and panics.
Hitsugaya, Bleach (attachments: Hinamori, his grandmother)
Yahiko, Rurouni Kenshin (Tsubame)
Hiei, Yu Yu Hakusho (Yukino)
Elmer, Rondo of Swords (Selmer)
Kaoru, Flame of Recca (Yanagi, Ganko)
Hiro, Fruits Basket (Kisa, his mother)
Wataru, Combat Butler Hayate (Isumi)
Elwood, Zombie Powder (his sister)
Sasuke, Samurai Deeper Kyo (Yuya)
Cassian, Godchild (none in particular. Jezebel?)
William’s next-younger brother, Emma (none I can think of)
Ed, Fullmetal Alchemist (Al) – Despite being the protagonist, Ed displays all the classic symptoms. Clearly, there really is an alternative universe in which the series is called The Fire Alchemist.
-Bizarro Universe Examples-
Syaoran, Card Captor Sakura (Sakura, Yukito) – Syaoran is quickly revealed to be bisexual and an excellent cook.
Yasuchika, Ouran High School Host Club (Honey) – Honey’s brother is initially set up to appear to be the sort of character who, due to his own social awkwardness, acts as if he hates his brother while secretly idolizing him. It is eventually revealed that he does, in fact, hate Honey.
(Further examples requested!)