A pair of Always Identical Twins with one stronger in some way (more intelligent, quicker, more athletic, etc.) and the other one weaker (less motivated, is the disappointment, inferiority complex).
The plot starts with the stronger twin dead or dying. The weaker twin mourns his brother's passing, but suddenly finds that he is thrust into the limelight and forced to Take Up My Sword and assume the mantle of his brother since he's a Suspiciously Similar Substitute. Often, this is due to his genetic make-up being exactly the same as his brother's, and hence, despite the apparent lack of suitability (his lack of confidence, weakness of character, or some other perceived weakness), he is the only possible candidate due to his genetic likeness.
Often the character suffers as the Angsty Surviving Twin but has no choice in the matter as he is the Backup Twin due to his DNA sequencing being exactly like his dead brother's and is essential to the plot moving forward.
Also, this is usually a perspective flip of Backup Twin shown from the perspective of the people who knew the original and received the back up. This is where the backup is the main character.
Examples:Anime & Manga
- In Basara, Sarasa disguises herself as her brother to fulfill his destiny for him after he dies. Genetics are not involved, but a prophecy that was told for both of them (but assumed to be about her brother) could be said to have the same function.
- Black Butler actually serves as a deconstruction of this trope. The protagonist is actually Ciel Phantomhive's twin brother and, while they have managed to achieve quite some things with, and without Sebastian's help, they are still weak in their initially weak areas.
- In K: Return of Kings, there's a scientist example with Adolf K. Weismann, who worked with his twin sister Claudia on the superpower-giving Slates decades ago - she died in a bombing, but the Slates made him immortal. He now has to solve an issue using his sister's notes on the Slates, and says that she was the smarter one, and that it's hard for him to understand her thinking. But he does end up figuring out what needs to be done.
- In Eagle Eye, Shia Lebouf plays a set of identical twins, but the main character is a 'failure' whose twin brother works on a top secret AI project called ARIIA. The brother was killed in an accident in the film. The main character is mysteriously 'activated' by the AI as his identical DNA is crucial to unlocking the AI from a state of inability to act.
- In Avatar, Jake is paralysed from the waist down; he survives his twin brother, who works on a scientific mission to Pandora and has a Na'vi avatar which only a genetic equivalent can use. Jake's asked to take up his brother's avatar (otherwise it's a waste of an investment), but instead, Jake sides with the Na'vi.
- In the Chris Rock vehicle Bad Company, a CIA agent dies in the middle of an important undercover operation, and the CIA call on his separated-at-birth twin brother, an immature hustler, to keep the cover identity alive.
- Once Upon a Time: In the first season episode "Shepherd," when Prince James is killed before he fights a Dragon so King Midas will reward his kingdom with gold, his twin brother David is quickly adopted and masquerades as him. Averted in that David was both a better fighter and prince than James.
- In the Robert A. Heinlein book Time for the Stars, the Long Range Foundation is sending Generation Ships to the stars, and to overcome the difficulties of time-dilated, luminal communication, they take advantage of the occasional ESP of identical twins. The hero lives in the shadow of his more out-going, adventurous brother, but ends up going to the stars when his brother breaks his leg skiing.
- Luigi's Mansion features a Lighter and Softer take on the trope, Mario, of course, isn't dead, or even hurt, but he is indisposed, and Luigi, who is more cowardly and timid than his heroic older brother, has to save him from King Boo. In Luigi's Mansion: Dark Moon, Luigi ends up having to save Mario again, along with the whole space-time continuum.
- One of the core themes of Mother 3. After Claus goes missing, Lucas is the only one left to pull the seven Needles and awaken the Dark Dragon. However, there's no mantle to take up: the mission is revealed long after Claus' disappearance. One instance where the fact that he's a twin is relevant is when Lucas is mistaken for the Pigmask Commander, granting him and his friends access into Thunder Tower.
- Sharp Zero has a variation of this. Elliot considers his brother Marcus the preferred twin and thinks of himself as the "evil twin" in the eyes of others. Marcus doesn't die, but Elliot is the one who gains superpowers.
- Gravity Falls had a variation of this. Dipper and Mabel's Great-Uncle Stanley was always The Unfavorite in comparison to his super-genius twin brother Stanford, who'd been introduced to the show in the mid-season finale. However, it's Stanley who ends up saving the day, by tricking Bill Cipher into entering his mind and then having Stanford erase his memories to destroy Bill once and for all.