Berserker's True Name is Penthesilea, an Amazoness queen in Greek Mythology. The daughter of Ares, starting from the eldest, Hippolyta, whose sash and life were robbed by Heracles, and her younger sister who was kidnapped and wed to Theseus, it is said that she has several more sisters.

After Hector's death, the Amazoness army led by Penthesilea went to assist the Trojans and fought against the Achaean army. And then, Queen Penthesilea ended up in a one-on-one fight against a hero on the enemy side, Achilles. The conclusion is the biggest humiliation at the end of her life.

The Amazon Queen who fought against Achilles during the Trojan War and was killed by his Diatrekhōn Astēr Lonkhē. Since then, because of an awful amount of regret when Achilles killed her with his spear, this had made him hesitant for him to fight against female opponents.

Supposing the grown-up version of herself from the Trojan War (generally 5-10 years later in terms of human body age) is summoned, it is believed that she can manifest in the Lancer, Archer, and even Rider classes.

(Source: Type-Moon Wiki)