Futakoi Alternative is a very loosely done Spinoff of the original Futakoi, which uses the original character designs and concept of twins and virtually nothing else. The male protagonist is Futaba Rentarou, a 21-year-old Private Detective who spends most of his time waiting for a case. He's accompanied by his twin assistants 15-year-old Shirogane Sara and Shirogane Sojyu, whose mutual affection for him is completely open to a threesome.
Genre Shift occurs almost by the episode throughout this show. One more serious example takes Rentarou from trying to resolve conflicted feelings of romance and responsibility to racing a bio-hybrid squid down a highway on a friend's bike in two adjacent episodes. The show does have an overarching plot, dealing with a mutual (and surprisingly non-rival) sibling love triangle.
- Art Shift: The endings are done in claymation reminiscent to shows like Postman Pat.
- Bears Are Bad News: A loan shark walks around with a polar bear for some extra muscle.
- Chase Scene: Several! Cars, motorcycles, a helicopter, a flying, fire-spitting squid, and shopping-kart are all used once or twice. Tropes Road Block and Excuse Me, Coming Through! also used.
- Childhood Marriage Promise: Between either Sojyu and Sara and their guardian, according to his father's will.
- Coming of Age Story: Literally for Sara and Sojyu, figuratively for Rentarou.
- Elegant Gothic Lolita: Twins Kira and Yura disguise themselves as "Gothic Lolita Mask" a few times in the series. The last time also involves gratuitous amounts of firearms.
- Fanservice: There's a Hot Springs Episode. Though fan service is not limited to one episode.
- Generation Xerox: A lot of people from town apparently thought that Rentarou would follow directly in his father's footsteps, and so calls him Nidaime (meaning "Second generation", or something) because of it.
- Good Cop/Bad Cop: Cop Hariyama is violent all the time, and his junior Kinoshita is gentle to a fault.
- Hand Seals: Ika fire mimics these before spitting fire once.
- Heel–Face Turn: Many! The Yakuza once the town's in danger of being leveled. Shimajima after he's won over by Rentarou's idealism. Kimihiko after he learns the truth.
- Love Triangle: Rentarou has a mutually loving relationship with Sara and Sojyu, but had a more traditional one with Ai and Mai.
- Polar Opposite Twins: Sara is impulsive, assertive, and resolute. Sojyu is calm, submissive, and kind.
- School Uniforms are the New Black: Sara and Sojyu are rarely shown wearing anything different besides what appear to be school uniforms. Hilariously, school is rarely even mentioned and never shown.
- Shout-Out: Hariyama's nickname is "Dirty Harry". He also takes down a helicopter with a revolver, referencing Jigen in the first opening sequence in the Lupin III: Part II series.
- Twin Threesome Fantasy: Sara and Sojyu would like nothing better, and Sara openly asks for it.