As advertised, Nier: Automata has a lot of endings. 26 in-fact. Each of the endings correspond to the letter of the alphabet, with a clever name associated with that letter.
Every ending is achieved through various decisions you make throughout the game, in addition to unique actions that sometimes require you to be within a third playthrough. With this, there is plenty of replay value that encourages that you start a new game with new objectives, and perhaps a higher difficulty.
Below are how you get all the different endings.
Ending A: flowers for m[A]chines
Complete the game for your first time.
Ending B: or not to [B]e
Continue from your save file after achieving Ending A.
Ending C: meaningless [C]ode
During your third playthrough (route C), choose "A2" during the final boss. Then, say "No" to POD's request during the credits.
Ending D: chil[D]hood's end
During your third playthrough, choose "9S" while engaged in the final battle before selecting "I'll go with you".
Ending E: the [E]nd of YoRHa
Continuing from endings C and D during a third playthrough, wait for the credits and accept the POD's request with a 'Yes'. An epilogue must then be completed.
Ending F: mission [F]ailed
During an ending B playthrough simply run out of time with 9S tries to restore his OS.
Ending G: hun[G]ry for knowledge
After achieving ending A, fail the mission when 9S arrives at the crane area.
Ending H: a mountain too [H]igh
Flee from the battlefield and fail the mission when the second Goliath is in the city.
Ending I: no [I] in team
While in Copied City during your first playthrough, defeat the boss, but run away from 9S afterward.
Ending J: bad [J]udgment
During Chapter 09-01_3 kill the "friendly" machines, including the Pastor.
Ending K: aji wo [K]utta
Eat the Mackerel fish given to you by Jackass at the Desert Camp.
Ending L: [L]one wolf
Leave the battle area to fail the mission during the final battle of a route A or B (first or second playthrough) run.
Ending M: break ti[M]e
Don't enter Pascal's Village to help during a third playthrough (route C) run.
Ending N: [N]o man's village
Slay villagers in Pascal's Village.
Ending O: just y[O]u and me
At the beginning of a third playthrough (route C), run away instead of engaging.
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Ending P: corru[P]tion
Allow 2B to die from the virus during a third playthrough (route C).
Ending Q: [Q]uestionable actions
Lose 2B as 9S during a third playthrough (route C).
Ending R: mave[R]ick
Destroy the friendly robots near Pascal in the Abandoned Factory.
Ending S: city e[S]cape
During a third playthrough (route C), run away from Popola and Devola when the tower hack sequence begins.
Ending T: fa[T]al error
Remove your OS Chip in the menus.
Ending U: deb[U]nked
Blow up the Bunker using Self-Destruct.
Ending V: reckless bra[V]ery
When fighting alongside Devola and Popola, wait until they run out of energy and don't hack the tower.
Ending W: broken [W]ings
Perish during the very first mission.
Ending X: time to rela[X]
When A2 and 2B meet at the commercial center, avoid helping 2B.
Ending Y: head[Y] battle
During "Emil's Determination" (secret boss), make sure Emil self-destructs.
Ending Z: over[Z]ealous
Kill Pascal after saving her when she is first met.
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