Plans for this sub-unit have been brewing since at least 2014 when Irene and Seulgi, oldest female trainees at the time, were paired for the "Be Natural"Performance Video belonging to the SMROOKIES Project. The group was officially announced in 2020. The duo's debut Mini Album Monster had one title track, plus one special track "Naughty" available only with the physical copy of the EP. Music videos were released for both.
- Irene (Bae Joohyun, born March 29, 1991) - Leader, Main Rapper, Lead Vocalist, Lead Dancer
- Seulgi (Kang Seulgi, born February 10, 1994) - Main Vocalist, Main Dancer, Maknae
- 2020 - Monster
Tropes that apply to the sub-unit and its music videos
- Abnormal Limb Rotation Range: Irene bends her back almost 90 degrees in heels for the "Monster" choreo.
- Big Little Brother: Gender Inverted. Seulgi is 3 years youger than Irene, but is the taller one.
- Contralto of Danger: The girls' lower registers are used a whole lot in the songs, and the unit has definitely one of the darkest concepts the group has ever done, if not the.
- Dancing Is Serious Business: Pride of the two main dancers of the group after all. "Naughty" is possibly the worst offender of this: they practiced the tutting and voguing of the choreography for 5 months to get it perfectly right. As soon as the music video was released, it was regarded as one of the most difficult performed by idols to date.
- Enemy Within: What the concept for the "Monster" music video boils down to. Irene is, likely, a demon that possesses Seulgi, commencing a Fighting from the Inside. In the end, it's apparently Seulgi that turns things around and gains control over Irene, becoming a monster herself.
- Hotter and Sexier: Although Red Velvet is no stranger to Fanservice, the Irene & Seulgi sub-unit takes the vampiness to a new level. (Justified Trope, because they're two of the oldest members and trainees since 2014 in general. It makes sense for them to have the "mature, sexy grown women" concept.) In "Naughty," the introduction scenes are:
- Queer Romance: The "Monster" music video is filled with Almost-Kisses. The aesthetic also seems to borrow heavily from the lesbian film The Handmaiden.
- Singer Name Drop: Downplayed. "Naughty"'s second-to-last verse has the words "Aseul, aseul~"("아슬아슬" in Korean, the first characters of Irene and Seulgi's names, which translates to "risky").
- Stock Sound Effects: "Monster" has distorted sounds of crying babies.
- Technician vs. Performer: Seulgi and Irene, respectively. Seulgi's style is sharp and angular, with powerful moves and explosive energy. Irene is more fluid and flexible, akin to a contemporary style. Needless to say, they're complementary to each other.
- Yin-Yang Clash: Considering the monochrome theme of the sub-unit, this paticular trope comes to mind.
- You Gotta Have Blue Hair: Seulgi dyed hers bright red for "Monster" promotions.