Which anime is Berserk from?

Berserk appears in Fate/Grand Order: Shuukyoku Tokuiten - Kani Jikan Shinden Solomon.

What role does Berserk have in that anime?

In Fate/Grand Order: Shuukyoku Tokuiten - Kani Jikan Shinden Solomon, Berserk is listed as a supporting character.

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Lu Bu is the Berserker-class Servant of Rani VIII. (Source: TYPE-MOON Wiki)

Fate/Grand Order: Shuukyoku Tokuiten - Kani Jikan Shinden Solomon

Fate/Extra

Fate/Grand Order: Dengeki Comic Anthology

Fate/Grand Order: Comic à la Carte

Fate/Ixtella

Fate/Grand Order: Fujimaru Ritsuka wa Wakaranai