The person impersonating another has to be so good at convincing others that they're who they're impersonating, they themselves are also fully convinced that they are who they say to be until they're outed.

Believing Their Own Lies: To qualify as a Pretender Servant, a simple disguise or alias won't just do.

For example Vortigern became Oberon thanks to Fairies of the sixth Lostbelt calling him such, to the point where his identity and being became Oberon for a time, while Hephaestion was used for Iskander's conquests in such a way where she became his shadow, essentially being part of his legend. Basically, one must fully pretend to be someone or something so much that they basically help inspire or strengthen the person/thing they are imitating, at which point they become the very thing they were imitating.

Becoming the Mask: According to Nasu, this is basically what qualifies someone to be a Pretender class Servant.

They deal extra damage against Sabers, Archers, and Lancers deal half damage against Riders, Casters, and Assassins and both deal and take extra damage against Berserkers. Pretenders are strong against Alter Egos, but weak against Foreigners, thus completing the fourth Tactical Rock–Paper–Scissors triangle in the game. The Pretender class was introduced in Chapter 6 of Cosmos in the Lostbelt, and was the second Original Generation class in Grand Order after the Foreigner class.

For example, an author who secretly takes up the pen-name of one of their dead peers and subsequently makes their works famous, leading to all the acclaim being directed towards the wrong person. In other words, a deception so complete that it fools even the deceiver themselves. To qualify, a mere pseudonym (such as acting under a mask or alias) does not suffice: it must be a complete misattribution that destroys the ability to assert between the 'real' and 'fake' identities. Pretender is a class for those who became famous under the guise of someone else, to the point where all the deeds that would qualify them as Heroic Spirits are attributed to a different person, erasing the 'individual' who was truly responsible.

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