The two participants in a yaoi relationship (sometimes also in yuri) are often referred to as seme (攻め or せめ) and uke (受け or うけ). These terms originated in martial arts and uke is used in Japanese gay slang to mean the receptive partner in anal sex. Aleardo Zanghellini suggests that the martial arts terms have special significance to a Japanese audience, as an archetype of male same-sex relationships involves that once held between samurai and their companions. Seme derives from the ichidan verb semeru (攻める to attack) and uke from the verb ukeru (受ける or うける to receive). The seme and uke are often drawn in the bishōnen style and are "highly idealised",blending both masculine and feminine qualities.
Zanghellini suggests that the samurai archetype is responsible for "the 'hierarchical' structure and age difference" of some relationships portrayed in yaoi and BL. The seme is often depicted as the stereotypical male of anime and manga culture: restrained, physically powerful, and/or protective. The seme is generally older and taller, with a stronger chin, shorter hair, smaller eyes, and a more stereotypically masculine, even "macho", demeanour than the uke. The seme usually pursues the uke, hence the name. The uke usually has softer, androgynous, feminine features with bigger eyes and a smaller build, and is often physically weaker than the seme. Zanghellini feels that these stereotypes come from shōjo manga conventions of depicting heroines and her female rival, where the heroine would be portrayed as kawaii and her rival would be portrayed as a sophisticated and adult beauty. When the characters were changed from female to male, these characteristics remained in the seme and uke characters. In this view, readers identify with the uke. Readers may identify with the seme, or the uke, or both at the same time, or instead become a voyeur.
Anal sex is a prevalent theme in yaoi, as nearly all stories feature it in some way. The storyline where an uke is reluctant to have anal sex with a seme is considered to be similar to the reader's reluctance to have sexual contact with someone for the first time. Zanghellini notes that anal sex is almost always in a position so that the characters face each other, not in the doggy style Zanghelli states is portrayed by gay pornography. Zanghellini also notes that the uke rarely fellates the seme, but instead receives the sexual and romantic attentions of the seme.
One stereotype that is criticized is when the protagonists do not identify as gay, but rather are simply in love with that particular person. This is said to heighten the theme of all-conquering love, but is also pointed to as avoiding having to address prejudices against people who consider themselves to have been born homosexual. In recent years, newer yaoi stories have characters that identify as gay.Criticism of the stereotypically "girly" behavior of the uke has also been prominent. It has been questioned if yaoi is heteronormative, due to the masculine seme and feminine uke stereotypes. Additionally, yaoi stories are often told from the uke's perspective. When the seme and uke roles are more closely adhered to, the uke character may be said to represent a "'vagina/anus' to be penetrated", but even as he is penetrated, his phallus is not forgotten, for example, as a seme simultaneously fellates and digitally penetrates his partner in Play Boy Blues. This combination of penetration and phallic pleasure reinforces depictions of sex in yaoi as challenging the idea that there is an active, penetrating, male sexuality as opposed to a passive, penetrated, female sexuality.
Though these stereotypes are common, not all works adhere to them. Mark McLelland says that authors are "interested in exploring, not repudiating" the dynamics between the insertive partner and the receptive partner. The possibility of switching roles is often a source of playful teasing and sexual excitement for the characters, which has been said to show that the genre is aware of the "performative nature" of the roles. Sometimes the bottom character will be the aggressor in the relationship, or the pair will switch their sexual roles. Riba, リバ (a contraction of the English word "reversible") is used to describe a couple that yaoi fans think is still plausible when the partners switch their seme/uke roles. In another common mode of characters, the author will forego the stylisations of the seme and uke, and will portray both lovers as "equally attractive handsome men". In this case, whichever of the two who is ordinarily in charge will take the "passive role" in the bedroom.
